V 



OP SOUTHERN INDIA. 97 



anteriorly, open at both ends ; beaks not nearly terminal; hinge of the right valve 

 with one direct tooth, convex anteriorly, truncated behind ; an internal rounded 

 direct rib commences under the cardinal margin, gradually becomes less prominent 

 and disappears towards the ventral margin." This genus was proposed for the 

 cretaceous Siliqua bipUcata, Con. If the existence of a single tooth in the ris-ht 

 valve can be considered as a permanent distinctive character, the separation from 

 Siliqua would have good grounds. The tooth is said to be broadest at the hinge 

 plate, and tapers to a very acute edge, which is expanded in the direction of the 

 shell's diameter, (Am. Journ. Conch., iii, p. 188). This peculiarity in the form of 

 the principal or cardinal tooth is often seen in species of Tagelus, Externally the 

 type species resembles the recent Fharella Javanica. 



11. Mesopleiira, Con., 1867. Conrad in his catalogue of the Solenid^ (Am. 

 Journ. Conch., vol. iii, pt. 3, 1867), places under this newly suggested name 

 Solen Udentatus, Spengler, Sol. javanicus, Lam., and Cult, suhteres, Conrad. 

 The last is left doubtful, but from the figure given of it in vol. vii of the Journal 

 Acad. N. Sc, Phil., (pi. 17, fig. 10), its dentition would appear very similar to that 

 of Solecurtus, The description of the dentition of Sol. javaniciis given by Lamarck 

 agrees with that of a Pharella. I possess a species from the Sundarbans, south- 

 east of Calcutta, which is very much like Sol. javanicus, and most probably iden- 

 tical with it; I found the same also very plentifully about JPenang in brackish 

 water. This species is, however, in its dentition, pallial sinus, &c., a true JPharella. 

 The third species, S. bidentatus, I am unable to make out satisfactorily from Chem- 

 nitz' figure in the new edition of his work. If it be, however, identical with 

 Say's Solen centralis, described in Journal Ac. Nat. Sc, Phil., vol. ii, p. 316, and 

 if this species is to be considered as the type of this genus, proposed under the 

 name of Mesopleura, it will only differ from Solecurtus by a short internal rib ^ 

 issuing below the beaks, proceeding obliquely and posteriorly towards the ventral 

 edge. It would thus form a connecting link between Ceratisolen and Solecurtus, 

 though very closely allied to the latter. 

 ^^\y^ 12. Solecurtus, m^iuY., 1825, (Fsammosolen, Risso, 1826; Macha, Oken, 

 1835, not 1815). Shell transversally elongated, the beaks are generally sub-central, 

 the ligamental fulcrum is large, anterior muscular impression elongated, pear- 

 shaped, posterior sub- quadrangular or roundish, pallial sinus deep, hinge with two 

 approximate teeth in the right valve, and two somewhat more distant in the left ; 

 of the latter the posterior one at the beginning of the fulcrum is sometimes 

 obsolete. 



Eorms which in their external appearance fully agree with recent species of 

 Solecurtus are abundantly known from the palaeozoic rocks, but they only become 

 sufficiently recognized from the Jurassic epoch upwards. Several of the upper ter- 

 tiary species, like S. strigilatus, &c., are the same as the recent ones. 



13. Azor, Gray, 1847, has been suggested for the recent Sol. coarctatus on 

 account of its smooth epidermis. The distinction is scarcely more than a sub-generic 

 one. But if all the forms possess the above character and have two teeth in the 



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