BRACHIOPODA. 



669 



turned into the hollow of the capacious and ventricose dorsal valve. 

 The species of Atrypa range from the Ordovician to the Trias in- 

 clusive, an exceedingly familiar and widely distributed species 



Fig. 532. — Atrypa reticularis. Silurian and Devonian of Europe and America. 

 (After Billings.) 



being the A. reticularis (fig. 532) of the Silurian and Devonian 

 rocks. 



The Silurian and Devonian genus Ccelospira is generally regarded as a 

 close ally of Atrypa. The brachial supports in this genus are of few 

 coils, and have the peculiarity that the apices of the spires are directed 

 towards each other. The Ordovician and Silurian genus Zygospira 

 is also nearly related to Atrypa, but the bra- 

 chial spires have their apices directed obliquely 

 into the cavity of the dorsal valve, and point 

 towards each other. On the other hand, in 

 the genus Dayia (fig. 533) the spires have 

 their apices turned obliquely into the cavity 

 of the dorsal valve in such a manner as to 

 point away fro7n each other, thus facing the 

 sides of the shell. The type of the genus 

 Dayia is the familiar Silurian Brachiopod 

 formerly known as Rhynchonella ?iavicula, 

 but now termed Dayia navicula. 



Family 6. Rhvxchoxellid^:. — In this 



rig- 533- — Interior of the dor- 



family the animal is free, or attached by a sal valve of Dayia navkuia, si- 



, , , . . r r lurian, enlarged. (After David- 



muscular peduncle issuing from a foramen son.) 

 beneath the beak of the ventral valve. 



The arms are spirally coiled, and are supported at their origins only 

 by a pair of short, curved, calcareous processes. The shell-struc- 

 ture is usually fibrous and impunctate, but in a few forms it is 

 tubulated. The shell is biconvex, usually with a curved hinge-line, 

 and a prominent pointed beak in the ventral valve. The geolo- 

 gical range of the family is from the Ordovician to the present 

 day, but the great majority of the genera are confined to the 

 Palaeozoic rocks. 



The type-genus of this family is jRhynchonella, in which the valves 

 are more or less convex, smooth, or plaited, and united by teeth and 

 sockets. The shell (fig. 534) is trigonal, generally with a sinus in 



