108 CRETACEOUS PELECYPODA 



3. Thyella, H. Ad., 1865, (Proc. Zool. Soc, Lond., p. 754). Shell rather in- 

 flated, sub-triangular, truncated posteriorly, surface desussated, striated ; cartilage in 

 an almost straight, projecting process; hinge in the right valve with two cardinal 

 teeth, of which the anterior is bifid, in the left valve a single one bifid ; no lateral 

 teeth. Th, pulchra from Singapore is the only species as yet known. 



b. Sul-:family,-^BmiELINM. 



The species of this sub-division chiefly bury in sand, and have very lon^ 

 siphons. 



4. Semele,^ Schuhmacher, 1817, (Amphidesma, Lam., 1818, apud 

 Deshayes and others). Shell oval, or sub-orbicular, hinge generally with two cardi^ 

 nal teeth in front of the narrow oblique cartilage groove, the anterior in the right 

 and the posterior in the left valve are usually smaller and sometimes become almost 

 obsolete ; lateral teeth close to the cardinal, well developed, but generally smaller 

 in the left valve ; pallial sinus ascending, wide, rounded at its termination. 



5. Syndosmya, f Recluz, 1843, {Abra, apud H. and A. Adams). Shell trans- 

 versally elongated, more or less attenuated posteriorly, thin, sub-equilateral ; hinge 

 in the right valve with two small cardinal teeth, in the left with one ; occasionally 

 all become obsolete ; lateral teeth usually distinct, but sometimes very small and 

 almost obsolete. 



The SyndosmycB are much thinner and posteriorly much more attenuated shells, 

 than species of Semele. 



6. Theora, H. and A. Adams, 1854. Shell transversally elongated, more or 

 less tumid, posteriorly attenuated, very thin, often hyaline, gaping posteriorly ; hinge 

 with two minute cardinal teeth in the right valve in front of an oblique posteriorly 

 directed cartilage process, one cardinal tooth, usually bifid in the left valve ; all the 

 cardinals become obsolete in some species ; lateral teeth distinct, anterior close to 

 the cardinal and in the left valve almost obsolete ; external ligament thin, but 

 sometimes supported by distinct fulcra, which are tooth-like, more thickened inside 

 than prominent above ; pallial sinus deep. 



The Theorm live in mud and sometimes brackish water. I have examined the 

 animal which has a strongly compressed, lingui-form foot, and two perfectly separated 

 long siphons, and thus fully confirms A. Adam's suggestion, that the genus 

 belongs to the '' Scrobiculariate division of the Tellinib^,'' 



* The name Amphidesma cannot be justly accepted in place of Semele. I may quote Philippi (Handb. der Conch., 

 p. 312,) on the subject; he says : "The name Semele not only has priority before Amphidesma, but deserves to 

 be accepted also for another reason, inasmuch as Amphidesma of Lamarck is a mixtum compositum of s-pecies' 

 of Semele, Zucina, Donacilla, Ahra, Thracia, Osteodesma, Kellia, &c." 



t In this case Recluz's name must, I think, have preference. It is true that Lamarck in 1818 quoted several 

 times the name Ahra under the synonyms of various species, but nobody knew, or could know, what Ahra meant, and 

 whether Lamarck intended retaining a genus called Ahra, and only transferring some of its species to Amphidesma, or 

 otherwise. Ahra as a generic name had not been characterized, so far as I know, nor in any way restricted to a known 

 type species before the introduction of Eecluz's name, and has, therefore, no claim to priority. 



