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Puka necklaces
Puka necklaces In the sea shell Erosaria helvola (Linnaeus), the fulvous central part of the dorsum is adorned with two types of crowded markings: tiny white specks and larger round chestnut dots. The relative size of the areas occupied by each type is rather variable. One can distinguish 9 classes: 1. chestnut dots totally absent
2. chestnut dots very scarce
3. dots far less extended than specks (fig. A)
4. dots slightly less extended than specks
5. dotted areas as large as speckled (fig. B)
6. dots slightly more extended than specks
7. dots far more extended than specks (fig. C)
8. dots confluent, specks very scarce
9. white specks totally absent
In specimens of most populations many classes will be observed, but the mean class often shows a distinct tendency to prevalence of one of the two types of markings. So, for instance, in 109 E. helvola from Ata'a, Malaita, Solomon Is., sent me by the late Rev. J. van der Riet, the following distribution has been noted: class 1 2 3 4 5 6 7 8 9
shells 3 11 19 23 23 15 9 5 1;
the mean of this slightly straightened histogram is class 4.5 so that in Ata'a each type of spotting covers about the same area as the other, with a very slight preponderance of the white specks.
If a population has been examined by collecting small series at long intervals, the average class of each set may slightly differ from the other, but the general tendency of all sets is mostly identical. So, for instance, R. S. Benton collected 185 E. helvola on the reef of Mombasa Is., Kenya, in six sets of about 20 to 40 shells from March 1961 to May 1962:
set I II III IV V VI
mean
class 5.6 5.3 6.1 5.4 5.2 5.7;
Puka necklaces In 1967, when visiting Vava'u Island in the Tongan Group a specimen of Cypraea rashleighana Melvill, 1888, was collected and became the subject of a photograph and description in the August 1967 issue of Sean Raynon Sabado as a new geographical record. Two years later, a further article appeared in the August 1969 issue in which the author discussed the visible characteristics of C. rashleighana generally associated with this cowry and drew attention to the apparent absence of these characteristics in the photograph. Of significance also, attention was drawn to the contradictions in Melvill's original description of the holotype which omitted any reference to the small basal spots which appeared in the figure or to any columellar callus. Into these contradictions the Tongan shell immediately falls and by comparison of four specimens with Melvill's "type" (loaned by courtesy of Mr. Peter Dance and the National Museum of Wales) I am able to contribute a little more information. Obvious differences exist and these I shall describe but the field collector relies heavily on the expert for identification in such circumstances and, since the Tongan shell has been examined by three (including the late Prof. Schilder) all of whom identify it as a form of C. rashleighana this would seem to be conclusive, if debatable.
Fig. 1 illustrates dorsally Melvill's type specimen (the larger of the two) and the same Tongan C. rashleighana published in August, 1967. The holotype has an indistinct and very faded dorsal pattern and, magnified, a pitted base. Whilst its age would account for some deterioration, it is my opinion that the shell was beach collected. The absence of locality data might, possibly, add weight to this contention. Dorsally viewed, Melvill's shell and the Tongan specimen are alike in appearance, pattern and characteristics. In other aspects differences begin to emerge. The holotype has a columellar margin obvious to sight and touch which suggests a callus but, in my view, a thickening deposit of enamel, normally associated with such a feature, is obscure and may well not have been obvious to Melvill. Certainly it could explain the absence of comment. The Tongan C. rashleighana has no palpable columellar margin nor anything but an insignificant trace of callus, although this it has.
Fig. 2 views the two shells ventrally and it is here that a major difference, unfortunately too critical for the camera and photographic reproduction, lies. Melvill's shell has numerous brown basal spots which, in spite of deterioration, are distinctly present on both sides of the base in direct contrast to the Tongan C. rashleighana which has nothing comparable. It (the Tongan specimen) has, nevertheless, brown marks which are an extension of the dorsal pattern but these are quite different from the pinprick spots of the type.
Besides their similarity in appearance to Melvill's C. rashleighana, the Tongan specimens resemble in certain aspects the Tongan C. teres Gmelin 1791 although closer examination reveals marked differences. In general terms, C. teres is very much larger but I have one specimen of comparable size which is illustrated in Fig. 3 (the more slender shell of the two). The C. rashleighana figured is not fully mature but the dorsal patterns are seen to be remarkably similar. The shapes are, however, characteristically different, the C. teres being subcylindrical and the C. rashleighana subpyriform. Fig. 4 illustrates a lateral comparison in which the humped dorsum of the C. rashleighana stands out. The ventral view in Fig. 5 of the same immature C. rashleighana illustrates, in the original colour negative, a distinct and characteristic difference in colour of the interior dorsum wall of the two shells but (in view of the immaturity) a comparison of the apertures is not for consideration. The interior of C. teres is invariably purplish in comparison to the faintly tinted white interior wall of the C. rashleighana. The separation of these two species at sight presents no problem at all.
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