1848. J SHARPE ON THE GEOLOGY OF OPORTO. 153 



Fig. 5 a. Dorsal valve. 



Fig. 5 h. Impression of ventral valve. 



The present shell is distinguished from all the published Lower 

 Silurian species of Orthis having simple ribs by the concavity of its 

 ventral valve. This is a rare character in the genus, and connects it 

 with O. semicircularis^ Eichw. and O. carinata, Conrad, Ann. Rep. 

 1839, p. 64. Nevertheless I do not feel sure that this may not ulti- 

 mately prove a variety of O. callactis, Dalm., a species which unfortu- 

 nately has never been published with sufficient detail. 



Orthoceras vagans, Salter, MSS. 



Smooth ; long tapering when young, more conical when old : septa 

 broad elliptical, oblique on the longer axis, moderately distant in the 

 young shell, distant by more than the diameter in mid-age, and by 

 less than one-fourth of the diameter when old, deep cup-shaped ; 

 siphuncle nearly central. 



This species differs from O. distans, Sow., in its smaller size, and in 

 the more central position of the siphuncle. O. interrwptus, Miinster, 

 is most like our species, but the septa grow more distant with age and 

 are more oblique, and it has a slight swelling of the shell between the 

 septa. 



This species is abundant in the Lower Silurian rocks of Wales and 

 Westmoreland. J. W. Salter. 



Found in slate at Vallongo near Oporto. 



Fig. 6 «. A young specimen. 



Fig. 6 h. One septum of an older shell. 



Bellerophon Duriensis, n. sp. 



Shell nearly globose, umbilicated, smooth ; aperture two-lobed, 

 divided in front by a deep angular sinus ; the lobes produced, their 

 outline nearly triangular with the apex rounded off. 



Diameter about 1 inch. 



Found in slate at Vallongo. 



This shell nearly resembles the B. bilobatus, Sow., from which it 

 differs in the angular form of the sinils, and the triangular outline of 

 the lobes of the aperture which are bounded by two lines nearly 

 straight, and only rounded at their extremities. It is also closely 

 allied to B. elongatus^ Portl. pi. 29. fig. 4. 



