IXTEODUCTIOX. 3 



Missouri country, Stoliczka in the first case, and Meek and Gabb in the others, had suffi- 

 cient supplies of excellent material, able investigators have not seldom been reduced to 

 the alternative of drawing conclusions from bad material, or of reaching no conclusions at 

 all. This consideration, it seems to us, has not had due weight m forming the verdict 

 which has been pronounced upon the work of Conrad as the first describer of Sj'riau fossil 

 shells in any considerable number. The collections of the Lynch Palestine Expedition, 

 and the other material which fell into Conrad's hands, were of very inferior quality ; and 

 if he were to name species, he was compelled to found them upon imperfect specimens, 

 for he seems to have had no others. It was his further misfortune that the descriptions 

 published in the Official Report of the expedition ai'e meagi-e beyond the habit of their 

 author, and that his figures, through fault of tlie artist, were poorly executed. From 

 these several causes has resulted uncertainty concernuig the identity of some of his 

 species, and respecting the validity of others. 



The SwLss Cretaceous Mollusca, described by Pictet and Campiche, afford a case not 

 unlike that with which Conrad had to deal. These authors have been justly criticised for 

 naming from casts so many species of the Veneridce, a family in which the shells have 

 often at the beaks and margins such thickness that it is impossible from the cast to re- 

 construct the exterior. But familiarity with the large Campiche collection (now in 

 the Museum of Comparative Zoology) of molluscan fossils from the Cretaceous of Sainte- 

 Croix, m which scores of specimens of the same species are but repetitions of like de- 

 fective casts, obliges me to think that, if species are to be discriminated in the fossils 

 of that locality, it must lie by means of imperfect casts, for, with few exceptions, better 

 examples are unknown. 



Again, m his Etudes Critiques sur Ics MoUusques Fossiles, — Monoi/raphie des Mi/c$, — 

 the late Professor Agassiz instituted, chiefly upon the basis of naked casts taken from the 

 Swiss Cretaceous strata, a series of new genera and species, of which the major part has 

 stood the test of later discovery and criticism. That distinguished observer saw that in 

 the family of Myidcc as limited by him (since in large part transferred to the Pholado- 

 rnyida- and Anatinida) the casts of his new genera indicated unmistakably that the shell 

 must have been very, thin, and that it was safe from the casts to infer the superficial 

 character of the shells themselves, and that " those features, which are included in the 

 terms general figure and ornaments of surface, acquu-e in the Mgida; [so limited] an in- 

 creased degree of importance from their invariable persistence and distinctness of design, 

 in a similar ratio that the hinges and their characters have degenerated in value." 

 (Morris and Lycett, Mollusca from the Great OuUte, Part II, p. 99.) 



So too among Gasteropods, by the introduction of the genus Tijlostoma Sharpe relieved 

 that of JSfatica from a burden impossible longer to be borne, and to-day are accepted as 

 valid, not only the genus, but the species, which he founded thirty-five years since on no 

 other basis than casts from the Cretaceous strata of Portugal, of which he says : " Few of 

 the specimens found retain any portion of the shell, and in no instance was I fortunate 

 enough to find a shell perfectly preserved, so that the specific descriptions are necessarily 



