INVERTEBRATE PALEONTOLOGY. 119 



thai il is really simple. The larger trigonal cardinal tooth of the left valve is 

 probably sometimes so deeply emargiuate as to give it an A-shape. 



The description and figures of the hinge of this genus have been made 

 out entirely from impressions left in the rock ; a rather difficult method, it is 

 true, but the only means we yet have of forming conclusions in regard to its 

 generic characters. Consequently, I have found it necessary, on farther 

 developing and more closely scrutinizing these moulds of the hinge, to modify, 

 to some extent, the description of the details of the hinge-teeth given in the 

 original description of the genus. What I at first supposed to be merely a 

 compressed, sharply-raised lamina, in the bottom of the pit behind the 

 trigonal anterior cardinal tooth of the left valve (see fig. 3, e, in which it is 

 represented a little too short, and not quite oblique enough), I now believe to 

 be properly the posterior cardinal tooth, and the oblique ridge above and 

 behind it, which I supposed to be the posterior cardinal, I now think a con- 

 tinuation, forward and upward, of the posterior lateral tooth, though it is not 

 represented exactly so in the figure ; while I now believe the thin projection 

 I here view as the posterior cardinal of the left valve, fits into a correspond- 

 ing slit just above and behind the posterior cardinal of the right valve, instead 

 of into the furrow along the middle of the same. 



Some time after the publication of the original description of this genus, 

 I had, through the kindness of Mr. Conrad, an opportunity to compare with 

 our type, perfect examples of the type of his genus Etea, showing the hinge 

 and interior of both valves as distinctly as we ever see in recent bivalves. 

 On first receiving Mr. Conrad's specimens, I saw that they closely resembled 

 this genus ; but being at the time occupied with other collections, no critical 

 comparisons were then made. On taking up the study of our Cretaceous 

 fossils again, and making careful comparisons of our shell with the typical 

 species of the genus Etea, I have been still more forcibly impressed with the 

 close relations of these types, and almost led to the conclusion that they may 

 not be generically distinct. The most essential differences that I have been 

 able to make out are the following: in the right valve of Etea* the lateral 

 teeth are shorter and double, instead of single, as they appear to be in Crassa- 

 tellina, the upper division of each being small, or merely rudimentary, while 

 the lower is prominent, and on the anterior side very nearly connects with 

 the base of the anterior cardinal tooth. 



* In making these comparisons, it is necessary to bear in mind that Mr. Conrad has inadvertently 

 described the hinge of the 1 ij;ht valve of his genus as that of the left, and the left as the right. 



