668 



MOLLUSCOIDEA. 



perforated by a foramen, below which is a deltidium. The shell-struc- 

 ture is punctated, and the brachial spires are similar to those of Spirifera. 

 The names of Trematospira and Rhy?ichospira have been proposed for a 

 number of Silurian and Devonian Brachiopods which are closely allied to 

 Retzia, but which differ from the typical forms of this genus in some 

 secondary characters. 



Lastly, the genus Uncites may be placed in this family, though 

 its characters are in some respects peculiar. The valves in Uncites 

 (figs. 527, b, and 530) are convex, radially striated, and of an im- 

 punctate structure. The beak of the ventral valve is very promin- 

 ent and strongly curved. The foramen in the beak of the ventral 

 valve disappears early, and there is no true hinge-area, but a large 

 concave deltidium is present, which partially conceals the incurved 

 beak of the dorsal valve. The margins of the beaks are bent in 

 wards, so as to form pouch-shaped expansions external to the hinge. 

 Well-developed brachial processes, of the form characteristic of the 

 Spiriferidcz, are present. 



The genus U7icites is confined to the Devonian rocks, the type being 

 the familiar U. gryphus of the Middle Devonian of Europe. 



Family 5. Atrypid^:. — In this family the shell-structure is im- 

 punctate, and the ventral valve has an incurved beak, with a curved 



hinge-line and no hinge-area. 

 The essential character of the 

 family, however, is that the 

 dorsal valve is provided with 

 two spirally-coiled brachial 

 supports, the apices of which 

 are directed towards the in- 

 terior of the valve (fig. 531). 

 The geological range of the 

 family is from the Ordovi- 

 cian to the Trias. 



The type of this family is 

 the genus Atrypa itself, in 

 which the shell (fig. 532) is 

 biconvex, generally radiately 

 ribbed, and often ornamented 

 with squamose lines of growth. 

 Though named in accordance 

 with the belief that the beak 

 of the ventral valve w r as imperforate, a small foramen is really pres- 

 ent in this genus (fig. 531), sometimes concealed, and sometimes 

 bounded in front by a small deltidium. The spiral brachial pro- 

 cesses are large and conical, and are directed with their apices 



Fig- 53 1 - — Atrypa reticularis, from the Silurian 

 rocks of North America, enlarged. The greater 

 part of the dorsal valve has been removed, to show 

 the loop and spiral brachial processes. (After Hall.) 



