119 



a crescent, marks the division between the cardinal area and the visceral Muscular 

 cavity. The middle of the cardinal area, at the hinge line, is three H ? ste of , 

 quarters of a millimetre be ow the sides, showing a depression here, as in 

 the ventral valve, but much wider. 



Muscle scars, Cardinal. Though there is a general depression at the 

 back of the valve in front of the cardinal area, there is nothing which 

 can with any certainty, be regarded as the imprint of a cardinal muscle. 

 But there is a pair of narrow scars outside of and behind the group of 

 lateral muscles that may have some office in connection with the hinge. 



Centrals. The group of central muscles is strongly marked and is 

 situated at the middle of the valve. The centrals or anterior adductors, 

 (A) are oval in form, and about as far apart as the width of the cardinal 

 area. The anterior laterals (j) are about as far in front of the centrals 

 as these are apart ; they also are oval; are smaller and are closer together 

 than the centrals. Between the two pairs of muscle scars of the central 

 group, along the median line runs a sharp medium septum, with a narrow 

 furrow on each side of it ; in front of the anterior lateral scars the sep- 

 tum is replaced by a furrow. 



Laterals. At the back of the dorsal valve is a kind of angulated cres- 

 cent, between which and the posterior end of the vascular trunks, the 

 group of lateral scars are crowded together. On the line of the crescent 

 on each side is a heavy oval scar (i) ; behind and outside of this is a 

 small narrow scar supposed to be lateral (or cardinal) ; in front of 

 this is a large scar, which like the one on the crescent is usually well 

 defined, this is supposed to be the (I) lateral ; an unimportant scar in 

 front of this, which is sometimes seen is also supposed to be a lateral (k). 



The vascular system. The main trunks in this valve are farther apart Circulatory 

 than in the ventral and more strongly arched. The branches also are 

 quite different in their course, etc. ; the main interior branches are 

 directed toward the central group of muscle-scars, and the outer ones 

 radiate regularly toward the margin of the valve ; the posterior inner 

 branches throw off several spurs each, on the posterior side. 



Both valves have sinuses in the visceral cavity, those in the ventral 

 narrow and angular, those in the dorsal broader, and rounded behind. 



That there are important differences between Obolus and Lingulella 

 will be seen from a comparison of the diagram of 0. Quenstedti given by 

 Michwitz with that of L. Selwyni as given above ; these may be tabulated 

 as follows : 



