GASTROPODA. 821 
Patellida.] 
represented among recent forms. The first of these specimens (a of the accom- 
panying cut) belongs to our new genus Archinacella; the second (fig. 1 6 and c) is of 
the type species of Tryblidium; while the third (fig. 1 d) appears to belong to an 
undescribed species of Lepetopsis. The first, then, is a Lower Silurian fossil, the 
second Upper Silurian, and the third Carboniferous. 
The recent Patellidw are distributed into generic groups according to the 
anatomical peculiarities of the animals, the characters of the shell being considered 
as of minor importance. Obviously such a rule cannot be applied to the fossil 
representatives of the family, seeing that their shells only are preserved. But that 
should 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 discriminating between the groups of species, and the 
systematist is 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 
the 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 
necessarily very limited. 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: 
TrysBiipium, Lindstrém. 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 and meeting in front beneath the beak. Surface usually marked by concentric 
lines of growth only; occasionally also by obscure broad radial plications. Type, 
T. unguis Lindstrom. 
ARCHINACELLA, n. gen. 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 forming a continuous band. Surface markings concentric 
only. Type, A. powersi, n. sp. 
Hetcronorsis, n. gen. General form and position of apex asin Tryblidium, from 
which the species differ in having the surface marked by fine radiating strie. 
Muscular scars unknown. ‘Type, H. fissicostata, n. sp. 
Patzacm@a, Hall and Whitfield. Shell forming a low cone, the base rounded 
or elliptical, the apex subcentral. Surface thrown into broad, rather regular, 
