XVIII SYNOPSIS OF FAMILIES 505 



Family. Strophomenidae 



Shell very variable in shape ; concavo-convex, plano-convex, or biconvex; 

 hinge line usually straight; frequently with an area on each valve; foramen 

 may or may not be present. Shell structure near always punctate. Ventral 

 valve usually furnished with hinge-teeth ; and dorsal valve with cardinal 

 process. 



Brachial supports completely absent or very rudimentary. 



(For muscular impressions see p. 502, Fig. 334.) 



Range. — AVholly Palaeozoic. 



Principal Genera. — Orthis, with many sub-genera, Clitambonites, 

 Skenidium, Sti'ophomena, Orihotheles, Leptaeiia, Stropheodonta, Plectambonites. 



Family. Koninckinidae 



Shell plano-convex or concavo-convex. Brachial apparatus composed of 

 two lamellae spirally enrolled in the same plane, or in the form of depressed 

 cones, with the apices directed into the ventral valve. 



Range. — Silurian to Lias. 



Principal Genera. — Koninckina, Koninckella, Coelospu^a, Davidsonia. 



Family. Spiriferidae 



Shell biconvex. Brachial apparatus consisting essentially of two 

 descending calcareous lamellae which by spiral enrolment form a pair of 

 laterally-directed cones (Fig. 330). 



Range. — Chiefly Palaeozoic, but a few forms pass up into the Lias. 



Principal Genera. — Spirifera, Cyrtia, Uncites, Athyris, Merista. 



Family. Atrypidae 



Brachial apparatus consists of two descending calcareous lamellae 

 which bend outwards at the extremity of the crura and are coiled into two 

 spiral cones, the apices of which either converge towards each other 

 (Glassia) or towards the dorsal valve (Atrypa, Fig. 332), or diverge towards 

 the dorsal valve {Day id) ; shell structure impunctate. 



Range. — Ordovician to Trias. 



Principal Genera. — Atrypa, Dayia, Glassia. 



Family. Rhynchonellidae 



Shell biconvex, hinge line usually curved. 



Beak of ventral valve incurved, with foramen. 



Calcareous brachial supports reduced to a pair of short curved crura. 



The septa, dental and socket plates may be highly developed and divide 

 up the cavity of the shell into chambers (Slenochisma, Conchidium). 



Shell structure fibrous, rarely punctate ; muscular impressions as in 

 Terebratididae. 



Range. — Ordovician to Recent : majority of the genera are Palaeozoic. 



Principal Genera. — Rhynchonella (Fig. 327), Slenochisma, Stricklan- 

 dia, Conchidium. 



