IPODA, 







represented aiinni^ n-r.-nt forms. Tin- first of these specim of the acn.m- 



panying cut i belongs to our new genus Ar> -Innacflln; the second (fig. 1 & and c) is of 

 the type species of Tri/Mi'lium; while the thinl (tig. 1 <f) appears to belong to an 

 undescribed species of /.'///>/.-. The first, then, is a Lower Silurian fossil, the 

 second I'pper Silurian, and the third Darboniferouv 



The recent l\i(rUi>ln are distributed into generic groups according to the 

 anatomical peculiarities ..f the animals, the characters of the shell being considered 

 as of mil. MI- importance. Obviously such a rale cannot be applied to the fossil 



sentatives of the family, seeing that their shells only are preserved. But that 

 hntiM not prevent the paleontologist from attempting a classification of the 

 numerous species that have been discovered in the rocks. The geologist requires 

 some convenient means of <li-< -riminating between the groups of species, and the 

 systematic i- in a great measure bound to respect this desire, especially if the 

 convenience of an arrangement does not directly oppose what he conceives to be 

 tli.- natural affinities. 



In arranging the Paleozoic species we have made use of the muscular scars 

 whenever these were available. But they are so seldom preserved that their use is 



--arily very limit. "1. In a large majority of the species the scars are entirely 

 unknown, and even under the most favorable circumstances they are generally 

 rather indefinitely outlined. We are, therefore, obliged to rely largely upon striking 

 external features, of which the form and outline, the position of the apex and the 

 character of the surface markings seem to be the most available. The groups may 

 be characterized briefly as follows: 



TI:YHUI>H M. Lind>trorn. Shell patelliform, obovate, narrowest anteriorly, 

 forming a very low cone; apex anterior, nearly marginal. Mucular scars in seven 

 or eight disconnected pairs, arranged in an oblong circle, the anterior pair drawn 

 out aip I meeting in front beneath the beak. Surface usually marked by concentric 

 lines of growth only; occasionally also by obscure broad radial plications. Type, 

 '/'. MfMM LiixUtn-m. 



n. jjen. Shell patelliform, ovate to subcircular, usually widest 

 anteriorly, forming a low cone with the apex in front of the center and often sub- 

 marginal. Muscular scars forminp a continuous l>an<l. Surface markings concentric 

 only. Type, A . n. sp. 



HKI.I IONMI-MS. n. ^ren. (uMieral form ami position of apex as in TnjMiilium, from 

 which the species differ in having the surface marked by tine radiating st 

 Muscular scars unknown. Tyi . n. -p. 



I'.M^ACiijBA. Hall and \Vhitfield. Shell forming a low rone, the base rounded 

 or elliptical, the apex subcentnil. Surface thrown into broad, rather regular. 



