Prof. F. M'Coy on new Cambro- Silurian Fossils. 391 



commencing a little in front of the ends of the diverging 

 lamellae, from between the ends of which project two long 

 spatulate muscular impressions, not reaching quite to the 

 middle of the shell. Width 2 inches 6 lines, proportional 

 length j^Q, depth of receiving valve ^o^, of entering valve 

 about the same. 



I have recently examined such a great number of specimens 

 from May Hill, that there can no longer be any doubt that it 

 belongs, not to the genus Spirifer as suggested by Sowerby, nor 

 to the genus Orthis as suggested by other palaeontologists, but is 

 a distinctly defined species of Pentamerus, distinguished by the 

 very small size of the rostral chamber in the receinng valve, and 

 the extreme shoxtness of the diverging lamellae in the entering 

 valve, which are not even indicated in ]\Ir. Sowerby's figm-e 

 thereof. As the species must be placed in the genus Pentamerus, 

 and there is already a Pentamerus lavis, I am obliged to propose 

 a new specific name. In very young specimens 3 lines long the 

 mesial septum exceeds half the length of the shell. 



In the fine sandstone of Mandinam, Caermarthenshire j very 

 abundant in the sandstone of ^lay Hill, Gloucestershire. 



{Col. University of Cambridge.) 



Hemithyris angustifrons (^I'Coy), 



Sp. Char. Elongate ovate ; both valves very gibbous in the mid- 

 dle, gradually sloping to the margins, which are nearly level, 

 or with a very slight elevation of the fi'ont towards the enter- 

 ing* valve ; greatest width in most specimens rather behind 

 the middle, from whence the width diminishes to the narrow 

 obtusely rounded front ; beak of the receiving valve large, 

 pointed, with a veiy long triangular opening beneath it ; sub- 

 stance of the shell coai'sely fibrous, surface nearly smooth or 

 with irregular transverse squamae of growth; casts show in 

 the entei'ing valve two subparallel approximate longitudinal 

 sulci mai'king the inner edge of the muscular impressions, and 

 with a fainter sulcus between them left by the slight mesial 

 septum ; two pits near the beak, left by the apophyses, strong ; 

 receiving valve with two strong dental lamellae, one on each 

 side of the beak, and a slight indication of a mesial septum ; 



* As the terms " dorsal and ventral valves " have been almost invariably 

 misapplied by conchological writers in describing Brachiojwds, and as the 

 confusion is verj- great when we use these terms in accordance with their 

 anatomical jwsition (but contrary to common use), I proi^se to use 

 "receiving valve" for the perforated valve of Terebratula, &c. (called 

 " dorsal valve " by conchologists and " ventral valve " by anatomists), and 

 I use " entering valve " for the opposite one, of which the beak enters the 

 cavit}- of the receiving valve. 



