BIVALVIA. 49 



extending nearly to the posterior extremity of the hinge line. Of the muscular impressions 

 the .anterior one is elevated upon a raised internal projecting ledge, as in Cucullaa, 

 the posterior impression is indistinct. The general figure is that of Byssoarca, the 

 umbones which are rather small, are placed near to the anterior extremity of a very 

 lengthened hinge line, the corrugation in the ventral border and hiatus are strong 

 points of resemblance to that genus. The dental characters present an approach to 

 those of Cucullcea, but in lieu of diverging from the central or subumbonal portion of 

 the hinge line, as in that genus, they are all turned in one direction inclining posteriorly. 

 Another external feature should be noticed, which it possesses in common with some 

 other of the Arcacea, viz. -. there is a depression upon the back of the shell, extending 

 obliquely from the umbo to the middle of the lower border. In generic value, this 

 form will take rank with Cucullcea and Byssoarca, but whether the two latter should 

 be regarded, of generic or only of subgeneric value, as considered by some authors, is 

 a subject which we will not discuss ; Palaeontologists, however, will perceive the conve- 

 nience of separating the present form from others of the Arcacea. 



Macrodon Hirsonensis, D'Archiac, Sp. Tab. V, fig. 1, \a, h, c. 



CucuLL^A ELONGATA. Phil. Geol. York., i, t. 11, f. 43, 1835. 



— — Goldfuss. Pet., t. 123, f. 9, 1840. 



— Hirsonensis, D'Archiac. Mem. Soc. Geol. Fr., t. v, t.27, f. 5, 1843. 



Testa in atate pmiori costatd, costis radiantibus, regularihus, et imbricatis, (ptate pro- 

 yrediente costis plerumque obsoletis, cum laminis incrementi magnis, paucis et rugis. 



Shell in the young state costated ; costae radiating, elevated, regular and imbricated ; 

 with increase of growth the costse gradually disappeared, and the surface was rendered 

 rugose by large folds or laminae of growth, which are usually few and distant ; they 

 become corrugated near to the hiatus in the lower border, as in Byssoarca. 



The aspect of this shell changed so much during the progress of growth, that without 

 a regular series for comparison, the larger and smaller specimens would probably be 

 separated into distinct species ; the figure given in the ' Geology of Yorkshire,' Pt. 1, 1. 11, 

 fig. 43, accurately represents the shell in its young state ; the costae are then sharply 

 defined, perfectly regular, and it has not acquired the laminae of growth which subse- 

 quently disarranged the regularity and continuity of the costae ; the figure of Goldfuss, 

 t. 12, fig. 9, is rather more elongated than is usual, it is of middle size, and the costae 

 are still visible; the figure of D'Archiac, t. 27, fig. 5, though beneath the middle size, 

 represents the stage of more advanced growth, in which the costae are obliterated, and 

 the lower border becomes corrugated. Sometimes, however, traces of the costae are 

 visible even upon shells of the largest size, and on the other hand, small shells may be 

 found smooth. The cast figured under the name of Macrodon rugosus, by Professor 

 Buckman, in the ' Geology of Cheltenham,' plate 5, fig. 5, appears to represent another 

 species which has a few distant and strongly-marked radiating costae. 



7 



