OP SOUTHERN IXDIA. 133 



Lycett (Suppl. Monog. on the Moll, from the Great Oolite, Palacont. See, 

 London, 1863, p. 65,) describes several species of this genus, adopting for them. 

 d'Orbigny's name SotDerbz/a, as a synonym of Isodonta. D'Orbigny (Prod. I, 

 p. 363,) suggested this name for a Jurassic species, S. crassa, of which he says that 

 it possesses a cartilage-pit. Lycett suggests that that author had likely seen only 

 tlie right valve and took the median pit, which corresponds to the tooth of the left 

 valve, as a cartilage-pit, believing, therefore, that the genus was allied to Mactra. 

 This may be, and probably is, the case, but as neither d'Orbigny nor, as far as I 

 know, any subsequent author had as yet given a description and figure of the type 

 species, it is impossible to accept his name in preference to, and as a substitute for, 

 Isodonta. It may after all turn out to be something else, and it is far preferable 

 to keep such uncertainties out of oiu' lists. Ko other but Jurassic species of 

 Isodonta haA'e been described, and they are few in number. The strongly 

 developed hinge-teeth and then* arrangements readily distinguish that genus from 

 Tanci'edia. 



3. Egerella, Stoliczka, 1870, ( Egeria, Lea, 1833, Contrib. to Geology, p. 49, 

 non idem Roissy or Leach). Shell elongated, sub-trigonal, anterior side much 

 shorter than the posterior; hinge with two cardinal teeth in each valve, one of 

 which is bind, lateral teeth none, sometimes they are indicated by a thickening 

 of the margins ; ligament external, apparently on the shorter side, inner edge of 

 shell occasionally crenated. 



Lea described several somewhat different species under this genus. Conrad 

 refeiTed the orbicular forms to ILjsia and Sphcerella, and they certainly belong to 

 the LucisiD.E, reserving the nSiVue Egeria for such forms asEg.subtrigona SiTidovalis 

 of Lea. These shells externally very much resemble the sub-genus Jia';Y< of TelUna, 

 but as the latter never have the inner margin crenated, it is probable that the 

 present classification of the genus is the more correct one. Conrad in his Check 

 list of eocene N. American fossils (1866) refers seven species to the genus. 

 Desh ayes and others describe similar tertiary forms, but we must yet wait for a 

 better characteristic in order to be able to distinguish the genus sufficiently from 

 Donax and several of its sub-genera. 



4. Donax, Linne, 1758. Shell ovately elongated, anterior side usually the 

 longer, wedge-shaped, rounded at the end, posterior shorter, obliquely truncated, 

 surface variously striated and covered with an epidermis ; hinge with two cardinal 

 teeth in the left and one in the right valve, lateral teeth stronger in the right than 

 in the left ; pallial sinus deep, broad and somewhat ascending ; inner edge of 

 shell smooth or crenulated. 



The genus Donax has been restricted by H. and A. Adams for the species 

 of the type of D. rugosiis, Linn., while some other forms resembling cotnjJressiis 

 have been i)laced by the same authors in the sub-genus Latona of Schuhmacher. 

 I confess that I am unable to find even a sub-generic distinction of any value 

 between the two divisions ; a few of the other sub-genera are, however, more 

 easily definable. Some of the species like D. vltiat/is ought to be referred to Sernila. 



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