216 CAMBRO-SILURIAN FOSSILS. 



small pointed slightly incurved beak, with a minute triangular 

 perforation beneath it, evenly gibbous except close to the sinus 

 in the front margin, where there is a very slightly marked de- 

 pression ; entering valve evenly gibbous, with a very slightly 

 marked, broad square ridge, extending a short way from the 

 margin of the sinus ; both valves radiated with from twenty to 

 twenty-four very obtuse, rounded ribs, becoming gradually 

 obsolete near the beaks, leaving the rostral portion nearly 

 smooth, four of the ribs rather larger than the rest, slightly 

 elevated with the mesial sinus ; a short mesial septum in the 

 entering valve ; substance of the shell densely fibrous. Length 

 5 lines, proportional width -^q^q, proportional depth of both 

 valves y'_o_. 



The depressed spheroidal form and very obtusely rounded 

 ribs becoming obsolete towards the beak, distinguish this spe- 

 cies from the H. nucula, and the elevation of the mesial sinus 

 being towards the entering instead of the receiving valve, as well 

 as more depressed form, distinguish it from the Terehratula sphce- 

 rica and Terehratula dejlexa of the Wenlock rocks, to which it is 

 otherwise very similar. 



This seems to be the Upper Silurian species referred to by 

 Mr. Davidson as identical with the Devonian Atrypa sphcerica of 

 Sowerby, to which it is very strongly allied, but is more de- 

 pressed, has only four mesial ribs, and the beak is not adpressed. 

 The obtuseness of the plaits and smooth rostral portion separate 

 it from the Terehratula pusilla (Sow.) of the Lower Silurian rocks. 

 The very obtuse margin, and smaller number of mesial plaits, 

 and obtuse inflated form separate it from the Terehratula famula 

 of Barrande (see Haidinger, Naturwissenschaftliche Abhandlun- 

 gen, Bandi. t. 17. f.6). 



Not uncommon in the Wenlock limestone of Dudley ; Ireleth, 

 Lancashire ; Wenlock limestone of Ledbury, Herefordshire j and 

 the limestone of the Hollies Church, Stretton. 



[Col. University of Cambridge.) 



Hemithyris subundata (M'Coy). 



Sp. Char. Transversely broad, oval ; valves gently and almost 

 equally convex ; beaks very small, apical angle 140° near the 

 apex, lateral margins straight, front -raised into a rounded 

 wave, from which, in the large valve, a wide shallow mesial 

 depression extends halfway to the beak, with a corresponding 

 elevation in the small valve, in some specimens faintly ex- 

 tended to the beak : surface smooth. Average width 1 inch, 

 proportional length f-^Qj length of entering valve y%%, depth 

 of both valves -^-^^ to y^Q%. 



