BRACHIOPODA. 
207 
often greatly elevated, especially in the plano-convex forms, and they are not 
usually produced into a ridge about the muscular area, but end abruptly. 
Muscular impressions quadruplicate, sometimes with radiating ridges extend¬ 
ing from the lateral and anterior margins. 
Shell-substance finely fibrous and punctate. 
This group has a large specific representation and a noteworthy vertical 
range. It appears to have had its inception in the Chazy fauna, Orthis subcequata, 
Conrad, being perhaps the earliest known representative. In the Tren¬ 
ton and Hudson River faunas are Orthis testudinaria and its close allies, 0. 
emacerata, 0. MeeJci, 0. multiseda, with a number of other species; in the Niag¬ 
ara, O. elegantula; in the Lower Helderberg, 0. perelegans, 0. concinna, 0. plano- 
convexa, 0. subcarinata; in the Corniferous, 0. lenticularis, Yanuxem, not Wah- 
lenberg; in the Hamilton, 0 lepida, and in the Chemung, O. superstes, sp. nov. 
With the close of the Devonian the type seems to have disappeared. 
There is a considerable difference in the external expression of the forms 
included in this group, and the plano-convex species like 0. elegantula, O. Wis- 
byensis, O. basalis, 0. planoconvexa, 0. subcarinata, which are readily distinguished 
from other members of the section. Some of the internal characters of these 
forms are also expressive; the flat brachial valve gives a great elevation to the 
crural plates, and the depth of the pedicle-valve makes the dental plates cor¬ 
respondingly high. These internal characters must necessarily vary in their 
development with the variation in convexity of the valves but it is doubtful 
if any subdivision of the group based upon these features could be of permanent 
value. 
In these species there is again occasionally found evidence of the secretion 
of an apical callosity in the delthyrium of the pedicle-valve, a feature which has 
been observed in Orthis perveta and 0. elegantula. Of additional interest is the 
peculiar crenulation or pectination of the inner surface of the high crural plates 
in Orthis elegantula, and some other species, a character highly developed in 
both teeth and sockets in Tropidoleptus and in Atrypa. 
