INVERTEBRATE PALAEONTOLOGY. 471 



1 ''"- ,>(i - On comparing this with 



( the septum of the form mulct 



uj consideration, it will l>e seen 



that, while there are other dif- 



A septum of Placenticeraswlalis , magnified to twine the fcrenCCS, tllC two agree ill the 

 natural size; drawn by \\ imam M. Gabb, irom Mortons 



original specimen, for comparison -with our figure at the top tripartite character of the first 

 of plate 23. , , .p. . . n 



lateral lobe, b rom this tact, 



and the nodiferons character of both, I was at first inclined to think that 

 the other differences might, to a great extent, be due to the much larger 

 size of the specimen from which the septum of our shell had been made out; 

 and, therefore, that the latter might perhaps with greater propriety be con- 

 sidered a variety of A. syrtalis. 



A specimen, however, from Texas, recently loaned to me by Prof. D. S. 

 Martin of Rutgers Female College, New York, agreeing in its noded periph- 

 ery and sides, and indeed in very nearly till other external characters, curi- 

 ously enough differs in having its first lateral lobe bipartite at the end, and 

 its septa throughout agreeing well with those of P . placenta, in shells of the 

 same size. The compressed nodes along the margins of the narrowly-flat- 

 tened periphery have the same alternating arrangement, and the little low, 

 rounded nodes near the umbilicus, also exactly as in the form here under 

 consideration. Another row of lateral nodes, however, on the form here 

 described, between the middle and periphery of each side, is very nearly 

 obsolete on the Texas shell. 



From all that is now known of these forms, I am still led to regard the 

 shell here under consideration as more properly a variety of P '. placenta than 

 of P. syrtalis. On comparing it, however, with the Indian form for which 

 Blanford proposed the name A. Tamulicus, but which Dr. Stoliczka figures 

 on plates xlvii and xlviii of his Palseont. Indica under Roemer's name A. 

 Guadaloupce, it will be seen that our shell agrees almost exactly in form and 

 other external characters, excepting that it shows no traces of the obscure 

 ridges seen on some, but not all, individuals of the Indian species. 



The septum of the latter, however, as made out by Dr. Stoliczka from a 

 smaller specimen than that of the form under consideration, figured on our 

 plate 23, shows marked differences in the form of its siphonal lobe, that at 

 least can hardly be due to the smaller size of the specimen. These differ- 

 ences consist in the much more slender branches of this lobe, and their 



