g8 Memoirs of the Indian Museum. [Vol. II, 



There are small differences in the other valves and in the cirri, but the relative 

 width of the scutum affords the easiest means of distinguishing between the two races. 



Genus DicheIvASPis, Darwin (1851). 

 Closely allied to Pœcilasma, from which its species may be recognized by the 

 imperfect development of their valves. These are never closely approximate. 

 Scutum never entire unless it is reduced to a mere rudiment, otherwise 

 consisting of two distinct segments or of two or even three branches united 

 at the base. Tergum as a rule with the scutal margin excavated to correspond 

 with the tip of the occludent segment or branch of the scutum, often much 

 reduced, sometimes absent. Carina simple and linear, forked at the base, 

 or expanded into a more or less well-developed transverse disk, occasionally 

 absent in some species. 

 When Darwin wrote his incomparable "Monograph" he experienced no great 

 difficulty in separating this genus from Pœcilasma, although he fully realised their 

 close relationship. Intermediate forms were not then known to exist. Several such 

 forms have, however, now been discovered and there is no longer any morpho- 

 logical justification for the separation. I retain the two genera as a matter of con- 

 venience, for together they would be of a somewhat unwieldy character. As Pilsbry 

 has recently pointed out, however, the name of those species which have imperfect 

 valves should, if the laws of priority were strictly followed, be Odolasmis, for 

 Gray described Octolasmis warwickii in 1825. Darwin was aware of the fact and 

 changed the name because it was inappropriate — a course that would not nowadays 

 be considered correct. To revive the earlier name, nevertheless, would only cause 

 confusion, for it has long been obsolete. 



The nomenclature of the Oriental species of the genus is at present in some con- 

 fusion owing to the large number of names that have been given to variable species. 

 Fortunately I have been able to examine considerable series, consisting in most cases 

 of over a hundred specimens, of all the species known to exist in Indian seas. I have 

 therefore been able to revise their synonymy with some confidence, although not 

 without prolonged deliberation in each case. In spite of the large number of specimens 

 examined, I have been forced to describe two species as new and to recognize eight 

 already described as distinct. One of these eight has only been recorded hitherto from 

 the North Atlantic. 



Key to the Indian species of Dichelaspis. 

 I. Tergum large, triangular or nearly so. 



A. Carina expanded into a large transverse disk: at the base. 

 a. Two segments of the scutum close to one another. 



Capitulum nearly as broad as long . . . . D. tridens. 



B. Carina neither forked at the base nor expanded into a 

 regular disk. 



h. Capitulum nearly twice as long as broad . . D. bathynomi. 



