54 PALEONTOLOGY 



description, though most that is said, apart from details of 

 shape and ornament, will apply equally to other species. 

 The shell is equivalve, the shape between oblong and 

 triangular, the posterior end truncated, the height about 

 two-thirds the length ; the umbo is near the anterior end 

 and opisthogyral. The greater part of the surface is 

 ornamented with tubercles, arranged in curved rows 

 oblique to the margin and tending to run together 

 anteriorly ; fine growth-lines can be traced over the 

 surface of the tubercles where these are not too worn ; 

 but from the umbo to the truncated posterior end there 

 extends an area in which the ornament is different, the 

 concentric striae being more marked, the tubercles much 

 smaller and in three radial rows. As it is the edge of 

 the mantle which secretes the shell, this difference in 

 ornament depends on a difference in the secreting activity 

 of the siphonal part of the mantle from that of the rest of 

 the mantle. Enclosed between this siphonal area and 

 the dorsal margin is a narrow segmental escutcheon, not 

 extending to the posterior end, and bearing only striae 

 whose obliquity to the margin shows that the shape 

 of this area has remained the same during growth. In 

 its extreme anterior portion, close to the umbo, is the 

 short but thick external ligament. This may be preserved 

 in the fossil ; if not, the position it occupied can easily be 

 recognized, when the two valves are in position, by the 

 gap between their margins. Being entirely behind the 

 umbo, the ligament is described as opisthodetic (in contrast 

 to the amphidetic ligament of Pectunculus.) Although 

 external, it does not act quite in the same way as that of 



