221 



Parazyga hirsuta. Hall. (Fig. 131.) (Pal. N. Y., Vol. 

 IV., p. 274.) 



Distinguishing Characters.— Length and breadth as three 

 to four; well-marked mesial fold and sinus, the sides of 

 which are not strongly defined ; grannlose surface, from 

 breaking off of spines. 



Found at Eighteen Mile Creek, in the Hamilton shales 

 (Hall), position not determined. 



Genus TREMATOSPIRA. Hall. 



[Ety. : Trema, foramen; spira, spire.] 



(1859: 12tkRep't N. Y. State Mus. Nat. Hist., p. 27. 1893: Pal. 

 N. Y., Vol. VIII. , Pt. II., p. 124.) 



Shells transverse, with sub-equally convex valves. Sur- 

 face radially plicate. Hinge line straight, cardinal extremi- 

 ties abruptly rounded. Anterior margin sinuate. Pedicle 

 valve with a median sinus and an incurved beak, truncated 

 by a circular foramen. Delthyrium covered by two short 

 incurved plates, which are usually closely anchylosed, and 

 appear continuous, with a narrow flattened false area on 

 either side. Lower half of the delthyrium open, for the 

 reception of the beak of the brachial valve. Teeth promi- 

 nent, arising from the bottom of the valve; above the hinge 

 line they curve backwards and towards each other, thus 

 making a very firm articulation. Muscular area well de- 

 fined. Brachial valve with median fold, and minute beak. 

 Hinge plate greatly elevated, with a small chilidium resting 

 against it; upper face of plate deeply divided by median 

 longitudinal groove, and more faintly by transverse groove. 

 Dental sockets small and deep, crura broad, thin and com- 

 paratively short. Brachidium of two spiral cones set base 

 to base, as in Spirifer. 



Trematospira gibbosa. (Fig. 131 A.) (Pal. N. Y., Vol. 

 IV., p. 273, PL XLV.) 



Distinguishing Characters. — Abrupt sinus and fold, especi- 

 ally in old shell ; nine or ten angular plications on surface ; 

 three small plications on fold, and two small ones in mesial 



