122 Memoirs of the Indian Museum. ^ [Vol. II, 



The series in the Indian Museum, which is a large one (including specimens 

 from the Persian Gulf, the Bay of Bengal and the seas of Sumatra and Java) exhibits 

 every gradation, so far as external characters are concerned, between the three forms 

 I have regarded as synonymous. Many of the Malayan specimens, moreover, agree 

 closely with Gravel's description of D. vaillanti. The nature of the variation exhibi- 

 ted within the limits of the species as a whole is precisely similar to that exhibited by 

 D. angulata, of which I have examined an even larger series. I propose to discuss 

 the question at considerable length in connection with the latter species, and it is 

 unnecessary to do so twice over. 



Dichelaspis angulata, Aurivillius. 



D. angulata, Aurivillius, op. cit., p. 22, pi. ii, figs. 9— 11, pi. viii, figs. 15, 24 



(1894). 

 D. aperta, id., ibid., p. 24, pi. i, figs. 14 — 16. 

 D. cuneata, id., ibid., p. 25, pi. i, figs. 17 — 19. 

 D. bullata, id., ibid., p. 26, pi. ii, fi.gs. 12, 13 ; pi. vi, figs. 10, 11 ; pi. viii, figs. 



19, 25. 

 D. transversa, Annandale , Ann. Mag. Nat. Hist. (7), vol. xviii, p. 44, figs, i, la. 



Minute, more or less transparent, colourless; all the valves linear; no tergum ; 

 the external surface naked or covered with small chitinous points. 



CAPITUI.UM rounded, more or less bullate, with prominent lobes above the aper- 

 ture, which is sometimes transverse. Tergum absent as a calcified plate, but often 

 represented by an amorphous chitinous patch, which is sometimes conspicuous owing 

 to its yellow colour. Carina sometimes absent, when present of variable size and 

 shape, often quite rudimentary, sometimes forked at the base ; the basal branches 

 never stout and never turned upwards. Scutum very variable, sometimes fully calci- 

 fied, forming a crescentic bar, often sinuous, occasionally angular, the lower extrem- 

 ity sometimes produced into a fine, imperfectly calcified basal branch, which is 

 occasionally separated from the vertical one ; the length of the basal branch, when it is 

 present, very variable. 



PeduncIvE variable in length, sometimes long, slender and cylindrical, some- 

 times short and stout, much broader at the base than at its junction with the capi- 

 tulum. 



Cirri, etc. — Cirri moderately long, curved; the hairs on their anterior margin, 

 which is markedly convex in each joint, long, becoming shorter towards the base of 

 each joint than they are at the apex, arranged on each joint in two vertical rows of 

 about seven hairs each. First pair of cirri short, not very widely separated from the 

 second ; the anterior ramus slightly shorter than the posterior, each ramus bearing a 

 circle of moderately stout bristles at the tip of the distal joint and otherwise profusely 

 covered with hairs. Anal appendages moderately broad, compressed, reaching at least 

 as far as the apex of the pedicel of the sixth cirri, bearing a dense tuft of moderately 

 long hairs on the tip and several groups of stouter and shorter ones on the posterior 



