BIVALVIA. 191 



the recent shell is that in the latter the hinge line appears rather thicker ; and the 

 same may be said of the Sicilian fossil. 



In this shell the sulci or ridges take their rise, and are most numerous, like the 

 two preceding species, on the posterior side, or that on which the ligament is placed, 

 where they are somewhat rugose, and descend or curve in an oblique direction over 

 the exterior or centre of the shell, and are lost or run out at different distances on the 

 ventral margin, most conspicuously so on the anterior portion. These rugosities are 

 regular on the posterior side, radiating in an opposite direction, like those upon the 

 posterior side of Visnus verrucosa, and are probably produced by the lines of growth 

 cutting the sculpture at an acute angle ; they are most distinct upon the Coralline 

 Crag specimens. 



There is a considerable variation in the outline of this species, some individuals 

 having a height as great, or greater, than the length, but in general the larger diameter 

 is from the anterior to the posterior margin ; there is also a difference in the sculpture, 

 the lines being much closer or more numerous in some than in others, and they are 

 always more distant near the umbo, or on the young shell, and most distinct upon 

 the Red Crag specimens. There are the remains of colour in some of my fossils, 

 like concentric bands. 



It was at one time thought there was sufficient difference in this species to entitle 

 it to more than specific distinction, and it remained in my cabinet under the MS. name 

 of Bigitaria vulgaris, and perhaps when the animal becomes known, it may present 

 characters that are so ; but judging from the shell alone, it so strongly resembles some 

 of the more aberrant forms of Astarte, that it seems more advisable it should be placed 

 here, bearing, as it evidently does, a greater relationship to this genus than either 

 to Tellina or Lucina, to which it has been hitherto referred. UnUke the former, it has 

 the impression of the mantle entire, or without any inflection, and the impressions of 

 the adductors are of an ovate form, the anterior one being rather the longer of the 

 two ; but unlike the latter, it wants the ligulate or band-shaped impression so charac- 

 teristic of that genus. The same obliquity of ridges may be seen upon a small species 

 with the hinge of Cardita, a genus probably very closely allied to the present one. 



19. Astarte excurrens, S. Wood. Tab. XVII, fig. 9(35. 



Spec. Char. Testa minima, transversa, ovatd, subcBquilaterali, compressiiisculd, striata, 

 striis obliquis utrinque excurrentibus ; umbonibus depressis, marginibus denticulatis. 



Shell small, transverse, ovate, nearly equilateral, somewhat compressed, covered 

 with striae that are not parallel to the margin, but run out from each side ; umbones 

 depressed ; margins denticulated. 



Largest diameter, \\X\ of an inch. 



Locality. Cor. Crag, Sutton. 



This shell is not by any means rare at the above locality. It differs from the pre- 



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