DITHYROCARIS. 133 



sides during life, and more easily separated into two parts after life, than in 

 the " Group A," or true Dithijvocaris. Thus : 



Grotjp a. 



Dithi/rocaris trieornis. 



— Colei. 



— ovalis. 



— testudinea. 



— Scouleri. 



Ghoup B. 



Ditkyrocaris graniilata. 



— glabra. 



— / tenuisiriata. 



— f Belli. 



We do not think that this suggested grouping is sufficiently well founded to 

 serve as a basis for classification. 



As seen in the Table at page 132, the features which characteristically define 

 some of the species, namely, Nos. 1 — 10, as truly belonging to Blthyrocaris, are 

 wanting in others, which therefore must be separated from that genus. 



The mesolateral ridges are present in all, in different degrees of development ; 

 but the dorsal ridge, an important feature, is absent in some (Nos. 11 — 15). 

 Some differ further by the two halves of the carapace not forming a shield-like, 

 but a bivalve test, folding down on each side, iu Nos. 12, 13, and 14, though not 

 perfectly fitting below, but remaining somewhat open; whilst No. 15, a costulated 

 form, is symmetrically bivalved and closed up, like some Ostracoda. 



Prof. R. P. Whitfield, at page 36 of the ' American Journal of Science,' 

 vol. xix, 1880, states ^ that " the genus Dithijrocaris, M'Coy, is described as 

 having three longitudinal ridges on the carapace. This feature is seen only when 

 the two valves are pressed open, as in M'Coy's example, so as to present the 

 appearance of one large plate, in which case the hinge-line forms the middle 

 ridge." 



This is also well shown in many of the illustrations of the present Monograph, 

 —such, for instance, as PI. XX, figs. 1 a and 3 a. There is, however, sometimes 

 present another longitudinal ridge iu each valve, lying near to and parallel with 

 the dorsal ridge. This, feeble in PL XXIV, figs. 1 and 2, strong in PI. XXI, 

 figs. 8, 9, and 11, and in PL XXXI, figs. 8 and 9, we term the juxtadorsal 

 ridge. 



At first sight the presence of this ridge might seem to constitute an important 

 difference ; but it is essentially present, though weak, in Ditkyrocaris trieornis and 

 D. Colei, and strong in a variety of B. insignis. It is also a characteristic of 

 Choenocaris tenuisiriata. It seems to be duplicated in Gh. Bichteriana, and is 

 recognisable among the costulse in Calyptocaris striata. 



Iu looking at the relative length of the Style and Stylets in the trifid caudal 



' Also at p. 365 of the ' Annals New Tork Acad. Science,' vol. v, 1890. 



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