INVERTEBRATE PALEONTOLOGY. 85 



which latter only is represented among the existing Mollusca, and but two or 

 three living species are yet known. These are found in the Chinese and other 

 eastern seas. 



The Palaeozoic, and some of the Triassic, Jurassic, and possibly a few 

 of the later species, referred to this genus, belong to Macrodon, or in part 

 to allied but distinct groups. The genus Macrodon is distinguished, from 

 ( 'uculleea by its more depressed, transverse form, more anterior beaks, longer 

 hinge, generally gaping basal margin, and particularly by having only one to 

 two or three straight, smooth, linear, posterior hinge-teeth, ranging parallel 

 to, and extending along two-thirds to three-fourths the length of, the hinge- 

 margin ; with a few very much shorter denticles at the anterior extremity of 

 the hinge, ranging obliquely forward and upward, without any intermediate 

 vertical denticles. 



Dr. Stoliczka refers two Indian Cretaceous species (Pal. Indica, III, 

 353 and 351, pi. xviii, figs. 6-11; and pi. xx, figs. 6-7) to Macrodon, and 

 doubtfully to Grammatodon, as a subgenus under the same, that seem to me 

 to be clearly distinct from both of those groups, and, as suggested by Mr. 

 Conrad, to be very closely allied to Idonearca. Grammatodon, as stated by 

 me in proposing it, is probably not more than subgenerically distinct from 

 Macrodon, as it agrees exactly in its hinge, and only differs from that type in 

 its shorter form, closed basal margins, and in having no raised lamina connected 

 with the posterior muscular scar.* Dr. Stoliczka's species differ decided/)/ in 

 their hinge-characters from both Macrodon and Grammatodon, and only differ 

 in the same from Idonearca, in having the short mesial denticles ranging 

 obliquely forward and upward, instead of vertically ; while their anterior 

 denticles, instead of ranging obliquely forward and upward, as in Macrodon and 

 Grammatodon, avc directed decidedly downward and forward. Their posterior 

 teeth .are also proportionally much shorter than those of the last-mentioned 

 groups. In short, the whole aspect of these shells is so nearly that of Ido- 

 nearca, and so unlike that of Grammatodon and Macrodon, that if they cannot 

 go properly into the first, surely they could only form another very closely 

 allied section of Cucullcea, and not of Macrodon. 



* Dr. Stoliczka doubts this last-mentioned character. I have only to say on this point, that no 

 traces of any biicIi internal processes arc to be seen on casts of the shells' interior. 



