CTENOSTREON PECTINIFORMIS — ETHERIDGE, 15 



the lateral margins, and the concentric lamella? continue longi- 

 tudinally across the posterior auricle, but the latter is destitute 

 of costal. 



Internally there is a well marked and broad area, traversed by 

 delicate cartilage gi'ooves, and divided medianally by a wide 

 chondrophore, which, slightly canted to the posterior, projects 

 below the lower margin of the area. The former is subconcentric- 

 ally grooved, but the grooves are coarser than those on the area 

 proper. The margins, anterior and posterior, as far as the trans- 

 verse median line of the valve are lamellar scobinate. Beneath 

 the somewhat projecting ventral end of the chondrophore, and a 

 little anterior to the latter, is a shallow hepatic fossa. The 

 adductor impression is moderately large, longitudinally oval, and 

 impressed on its posterior side, so that in casts of the interior, a 

 semi-oval ridge would be left. 



There appears to be a large amount of variability in the number 

 of costfe decorating the exterior of Ctenoslreon j^ectini/ortnis. 

 Goldfuss figures a wide shell with fourteen highly fistulous costse; 

 Bronn a medium sized mollusc with eleven, and an abnormal, 

 long, narrow individual with equal pectinoid auricles, possessing 

 six costse. The shell figured by Morris and Lycett has ten costte; 

 and, as previously stated, the West Australian specimen now under 

 description exhibits nine normal and well developed ribs. An 

 example from Olschingen, in our Collection, possesses eight costie, 

 and another from Neuhausen twelve. 



A similar variability exists in the form of the valves, and the 

 extent to which the auricles are developed. In the present example, 

 and in one from an unknown German locality, the anterior auricle 

 is entirely wanting; in a second individual from the latter place 

 there is a slight anterior auricle ; whilst in the shell so beautifully 

 illustrated by Goldfuss, and in Morris and Lycett's Great Oolite 

 specimen both auricles are well grown. 



The chondrophore varies in longitudinal diameter, but its pos- 

 terior obliquity seems to he fairly constant in all that have 

 come under my notice. The chondrophore of the recent Malleus 

 malleus, Linn., is precisely similar to that of the present shell, 

 both as to form, direction, and extension below the cardinal area. 

 That of Meleagrina margaritifera is on the same plan, but much 

 more transversely drawn out. 



The adductor impression is also a variable feature, longitudinally 

 oval or round, and in two out of three valves now before me, 

 deepest on the posterior side. 



The hepatic fossa, although constant in all the specimens ex- 

 amined, is neither large nor deep. I use this name to distinguish 

 the pit so generally found penetrating the umbonal cavity of both 

 valves, or only the right valves of aviculoid shells. Amongst 



