ifODi ' 1 7 



PUuroUMMtrlld* ) 



h year the absolute necessity of extending our systematic paleontologies! studies 

 backward as far us possible becomes more and more obvious. The rapid changes 

 which took place among the early representatives of all olaMOB of animals and the 



con-.-. (in-lit relati\e ea-e \\itli \vliirli the lines of evolution may In- traced. j_'i\e- 



them an importance in Biology that is scarcely to be overestimated. For the 

 ' he Lower Silurian species, because the class is but sparingly represented 

 in the <'aml>iiaii, are the most likely to throw light upon the genetic relations of the 

 succeeding forms, and are therefore deserving of the most careful investigation. 

 Concerning the Heurotomnriidn; the facts brought out in an extended study, of the 

 1'aleo/oic -pecies chiefly, have led us to conclusions that, while not greatly different 

 from those publish^! i>\ Koken, who has adopted methods more nearly in accordance 

 with our own than any previous observer, are still sufficiently original to cause 

 great changes in the views and nomenclature heretofore in vogue. We have gone 

 into the subject more extensively than the present work demanded or perhaps even 

 ju-titied. hut the knowledge gained, if it cannot all bear fruit immediately, is -till 

 not in vain, -ince it will doubtless prove of use in our future work. We have most 

 carefully considered the published papers of authors who have dealt with Paleozoic 

 <iinriiil,r, particularly those of De Koninck, Sandberger, LindstrOm, (Ehlert 

 and Kok.Mi. and it would please us greatly to enter into a detailed account of their 

 various views. Hut as this would require more space than we have at our disposal 

 and would moreover be out of place in a work of this kind, we are obliged to post- 

 pone it to some more fitting occasion. Incidentally, however, we shall frequently 

 refer to them, especially when our opinions happen to differ. 



We believe it is admitted generally that the essential feature of the Plettro- 

 tomariidi , ami the peculiarity relied upon chiefly in distinguishing the family from 

 other spiral shells, excepting of course the symmetrically enrolled bellerophontids 

 and certain EuampMrnKda, / Y>-M< ;<///</'. TurrHeUiiln and Cerithiida; is a definitely 

 limited narrow Land, terminating anteriorly in the bottom of a more or less deep 

 sinus of the outer lip merely, or in a long, open and sometimes periodically closed 

 >lit. The Ion- parallel-edged slit occurs, as far a- known to us, in but two Lower 

 Silurian species (.sV/M.-o/oy,/,,; of tin- work), and is comparatively rare among the 

 I pper Silurian and Devonian form*, l.ut with t lie Carboniferous species it is common, 

 while among the more recent form- it i- nearly always present. 



Of all the char. f the family, the peculiarities of the <lit and hand furnish 



us with the most reliable grounds upon which to base our generic divisions. Accord- 

 ing to these the family might be divided primarily into two principal groups, the 



having a sinus only in the outer lip, the second a long parallel-sided >lit or a 

 series of openings. These groups are again divisible, according to the character of 



