1849.] SHARPE ON THE SECONDARY ROCKS OF PORTUGAL. 183 



From subcretaceous limestone between Sobral and Torres Vedras, 

 and also about three miles south-west of Alenquer. 



This shell has some resemblance in form to Astarte suhtrigona, 

 Goldf., and to Cytherea trigonellaris, Voltz, from both of which it 

 is readily distinguished by the steepness of the dorsal side and more 

 triangular outline. The muscular impressions are small. 



DiANCHORA ? BTCORNIS, n. s. Pl. XXVI. fig. 4 & 5. 



Shell irregularly gibbose ; variable in form ; attached ; beaks of 

 both valves considerably produced ; shell very thin and fragile ; both 

 valves covered with fine, close-set, nearly equal, rounded ribs, which 

 are crossed at irregular intervals by distinct concentric lines of growth. 

 Hinge 1 



Length 1| inch. 



In beds of limestone of the oolitic series, above the coal, at Cape 

 Mondego. 



This curious shell is evidently related to the Dianchorce, Sow., 

 which have been placed by Deshayes, Goldfuss and D'Orbigny in the 

 genus Spondylus upon insufficient evidence, as their hinges have not 

 yet been seen. They differ from the true Spondtjli in their thin fra- 

 gile shells, the interior of which shows all the markings of the exte- 

 rior ; these contrast most remarkably with the thick solid shells of 

 Spondylus, which are lined internally with constantly accumulating 

 deposits of shelly layers. Deshayes has suggested as a solution of 

 this difficulty, that the inner thick layers of these species decayed 

 readily, leaving only the outer thin layer preserved. This is a mere 

 assumption unsupported by any evidence, and is in the highest degree 

 improbable, as they are found in various strata and in different coun- 

 tries in the same state of preservation, and uniformly with only a thin 

 shell. 



The hinges of my specimens are broken, as is usually the case in 

 this and other species : this seems to mark some peculiarity which has 

 not yet been explained. Perhaps the animal fixed itself to one body 

 by a byssus passing through the hinge-area, while the shell was ce- 

 mented by calcareous matter to any other body which touched it, and 

 the shell may have broken off, leaving the hinge attached by the 

 byssus. The Spondyli also appear to have been attached by a byssus 

 through the hinge-area when young, but to have dropped this connec- 

 tion as they grew older and closed up the aperture through which 

 the byssus passed, the trace of which remains in the suture down the 

 middle of the hinge-area of the lower valve. 



The different specimens of J), hicornis vary much both in size and 

 form ; the latter seems to have depended in a great degree on the 

 size and position of the body to which they attached themselves. 



Plate XXVI. fig. 4. An old specimen. 



Fig. 5. Side view of a young shell. 



DiCERAs Favri, n. s. Pl. XV. fig. 3 c, 3 6, & 3 c, and Pl. XX. fig. 9. 



Shell irregular, with very dissimilar valves ; the larger valve at- 

 tached when young, spirally twisted, divided longitudinally in nearly 



o 2 



