BRACHIOPODA. 



beak to the anterior part of tlie valve. Dorsal valve depressed 

 ■udthout an area, and with, a distinct mesial fold. Sliell-structure 

 punctate. 



Fossil, 2 species. Carboniferous. United States, Ireland, 

 Belgium. 



Cyethsta, Davidson, 1858. 



Etymology, modified from the diminutive [Cyrtidium) of 

 Cyrtia. 



Examples, C. beteroclyta, C. Demar- 

 lii, and C. septosa. 



Shell resembling Spirifera, but with- 

 out the vertical shelly plates which 

 diverge from the extremity of the beak. 

 Interior of ventral valve with two con- 

 tiguous vertical septa, which coalesce 

 into one median plate, which extends 

 from the extremity of the beak to within 

 a short distance of the frontal margin, 

 and then diverges to form dental plates, , F^so. 



as in Pentamerus. The fissure is co- s, sIpSm f ?f DetS' piatet? rf', 

 vered by an arch-shaped deltidium ; deitidium ; x, v-shaped chamber, 

 but in C. Demarlii the median septum is continued as far as 

 the under surface of the deltidium, and the dental plates are 

 fixed to the sides, instead of the upper edge, as in O, heteroclyta 

 and 0. septosa. 



' ' Spiral coils having the same position as in Spirifera, but 

 the two first coils are connected a little in front of the mid- 

 length by an apparatus somewhat like that of Spirigera, but 

 not so complicated. A very slender process springs upwards 

 towards the ventral valve from each coil, and, at a height of 

 about one line, curves forwards. The two then unite and form 

 a single band, which extends forwards to about the front of th 3 

 coil, and there ends in an obtuse point." — (Billings.) 



Distribution, 9 species. Devonian- — Trias. Europe and 

 North America. 



Meristella, Hall, 1860. 



Etymology, diminutive of Merista, an allied genus. 



Examples, Atrypa tumida, Dal. ; Meristella Isevis, Hall. 



Shell oval, ovoid, orbicular or transverse. Valves unequally 

 convex, with or without a median fold and sinus ; beak appa- 

 rently imperforate, incurved ; area none. Surface smooth or 



55 



