ATRYPID^. 319 



depressed in front ; beak small, often closely incurved ; foramen 

 round, sometimes completed by a deltidium, often concealed ; 

 dorsal valve with a divided hinge-plate, supporting two broad 

 spirally coiled lamellfe ; spires vertical, closely appressed, and 

 directed towards the centre of the valve ; teeth and impressions 

 like Rhynchonella. 



The shells of this genus differ from Rhynchonella chiefl,y in 

 the calcification of the oral supports, a character of uncertain 

 value. 



The internal appendages of Atrypa reticularis consist of a 

 pair of spiral cones, placed side by side, with their apices directed 

 towards the cavity of the dorsal valve ; the lamellae have their 

 origin on the socket-walls, and run parallel with the inner margin 

 of the valve. " The spiral cones are connected by an entire and 

 continuous loop, which is confined to the rostral part of the shell. 

 The loop ai-ises from the posterior portion of the first volutions 

 of the spires, and curves gentlj^ forward and upward ; the central 

 or elevated portion is situated between and behind the cones, 

 and forms a more or less abrupt curve, or is prolonged into a 

 point directed towards the dorsal valve. The existence and form 

 of this loop have been ascertained in several different varieties 

 of A. reticularis^ as well as in A. spinom, Hall." — Whitfield. 



ccELospiRA, Hall, 1863. Shell fibrous, concavo-convex; spiral 

 lamelli^ with their bases turned towards the ventral, their points 

 towards the dorsal valve. A. Camilla, Hall (cxxxvii, 92 ). 



STENOscHiSMA, Hall, 1847. (Zygospira, Hall, 1862.) Spiral 

 cones connected b}^ an entire and continuous loop in a very 

 similar manner to that shown to exist in Atrypa reticularis ; but 

 the loop having its connection with the spiral lamellae at a point 

 relatively more distant from their origin on the hinge-plate, and 

 passing over or in front of the spires. A. Tnodesta, Say. Silu- 

 rian ; IJ. S. 



Glassia, Davidson, 1881. 



Distr. — 3 sp. Silurian; England. G. Whidbornei, Davidson 

 (cxxxvii, 93). 



Principal lamellae, forming the first coils of the spirals, con- 

 nected at a short distance from the attachment to the hinge-plate 

 by a ribbon-shaped lamella or loop ; this loop, commencing on 

 each side from the principal lamellae, converges downwards in 

 the shape of the letter V ; principal coils of the spirals directly 

 face the lateral margins ; the ends of the spirals meet each other 

 in the centre of the shell ; each spiral consists of four coils. 



Anazyga, Davidson, 1882. 

 Distr. — A. recurvirostra, Hall (cxxxvii, 94). Trenton lime- 

 stone, Canada. 



Shell small, longitudinally oval, radiately striated ; position 



