SYNOPSIS OF FAxMILIES 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 furnislied 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. — Wholly Palaeozoic. 



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

 SJcenidium, Strophomena, Orthotlietes, Lcptaena, Stropheodonta, Pledambonites. 



Family : Konincldnidae 



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. — KonincJdna, Koninckclla, Coelospira, Davidsonia. 



Family : Spiriferidae 



Shell biconvex. Brachial apparatus consisting essentially of two descend- 

 ing 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, Gyrtia, Uncites, Athyris, Merista. 



Family : Atrypidae 



Bi'achial 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 (Dayia) ; shell structure impunctate. 



Range. — Ordovician to Trias. 



Principal Genera. — Atrypa, Dayia, Glassia. 



Family : Ehynchonellidae 



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 highh' developed and divide 

 up the cavity of the shell into chambers {Stenochisma, Conchidium). 



Shell structure fibrous, rarely punctate ; muscular impressions as in 

 Terebratulidae. 



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



Principal Genera. — Rhynchonella (Fig. 327) Stenochisma, StricJclandia, 

 Conchidium. 



