BRACHIOPODA. 207 



often greatly elevated, especially in the plano-convex forms, and they are not 

 usually produced into a ridge about the muscular area, hut 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 suhczquata, 

 Conrad, being perhaps the earliest known representative. In the Tren- 

 ton and Hudson River faunas are Orthis tesiudinaria and its close allies, 0. 

 emacerata, O. Meeki, 0. muUiseda, with a number of other species; in the Niag- 

 ara, 0. elegantula ; in the Lower Helderberg, 0. perelegans, 0. concinna, 0. plano- 

 convexa, 0. subcarinata; in the Corniferous, 0. lenticularis, Vanuxem, not Wah- 

 lenberg ; in the Hamilton, O lepida, and in the Chemung, 0. 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 O. elegantula, 0. Wis- 

 byensis, 0. 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. 



