MOLLUSCOIDA I BRACHIOPODA. 3 I 7 



pair of short, curved, shelly processes. Shell-structure fibrous and 

 impunctate. 



Genera. Rhynchonella, Pentamerus, Porambonites, &c. 

 Fam. VI. Strophomenida. 



Animal unknown ; some probably free, others attached, during the 

 whole or a portion of their existence, by a muscular peduncle. No 

 calcified supports for their arms. Shell with a straight hinge-line, and 

 a low triangular area in each valve. Shell-structure fibrous and 

 punctated. 



Genera. Orthis, Orlhisina, Strophomena, and Lept&na, 

 Fam. VII. Productida. 



Animal unknown. Shell entirely free, or attached to marine bottoms 

 by the substance of the beak ; valves either regularly articulated, or 

 kept in place by muscular action. No calcified support for the oral 

 appendages. 



Genera. Producta, Chonetes, Strophalosia, Aulosteges. 

 Fam. VIII. Craniada. 



Animal fixed to submarine objects by the substance of the shell of 

 the ventral valve. Arms fleshy and spirally coiled ; no hinge or 

 articulating processes ; upper or dorsal valve patelliform (i.e., limpet- 

 shaped). 



Genus. Crania . 

 Fam. IX. Discinida. 



Animal attached by means of a muscular peduncle passing through 

 the ventral or lower valve by means of a slit in its hinder portion, or 

 a circular foramen excavated in its substance. Arms fleshy, valves 

 unarticulated. 



Genera. Discina, Trematis, Siphonotreta, Acrotreta. 

 Fam. X. Lingulidtz* 



Animal fixed by a muscular peduncle passing out between the beaks 

 of the valves ; arms fleshy, unsupported by calcified processes. Shell 

 unarticulated, sub-equivalve, texture horny. 



Genera. -J^ingula, Obohis. 

 Fam. XI. Trimeretlidcz . 



Shell calcareous, not horny ; hinge-area well developed ; deltidium 

 wide, bounded by two ridges, the inner edges of which serve as teeth ; 

 limbs generally very prominent, usually massive, and often divided by 

 a partition into two chambers. Each valve with muscular platforms, 

 which are typically elevated and vaulted. 



Genera. Trimerella, Dinobolus, Monomerella. 



* Another family was formerly constituted for the reception of the sin- 

 gular Devonian fossils known as Calceolce. It has been shown, however, 

 that these are probably operculate Corals. 



