12 Prof. W. King on some 



simply attached by its inner edge, now widened, to the narrow 

 space between the root of the pedicle and the postparietal. 



In the pleurocoeles the inner edge of the setal band corre- 

 sponds to some extent with the line formed by the main 

 posterior branches of the vascular system ; consequently both 

 structures contribute to form the outer boundary of these 

 chambers. In the brachiocoele the same edge {v) is in im- 

 mediate connexion with the line formed by the constriction of 

 the outgoing secondary vessels {v) ; and it becomes thicker and 

 more prominent, appearing as if irregularly frilled, in passing 

 to the front of the valves. The rapidly attenuated vessels, 

 previously noticed, cross obliquely, with a slight forward 

 curve, the setal band, giving it, especially its inner edge, a 

 somewhat plicated character. The band is also crossed by 

 what appear to be incised lines, or stride, directed less obliquely 

 than the latter vessels, and which become subdivided near its 

 outer edge. 



In none of the specimens of Lingula anatina that have 

 passed under my observation is there any well-marked im- 

 pression of the setal band. Aged individuals with thick 

 valves very probably show something of the kind ; and it can 

 readily be imagined that in such the muscular attachment of 

 the inner edge of the band will produce a submarginal 

 scar running round both valves. Usually the posterior half of 

 the valves is the thickest : and it is on the outside of the 

 spaces that represent the pleurocoeles (belonging to this 

 half) that impressions may occasionally be observed, con- 

 sisting of an obscurely defined line that runs backward 

 from each side of the splanchnocoele, where it is widest, to 

 the hinge. Traces are seen of in-egular impressions (which 

 answer to the offshoots) striking from both sides of the above 

 line, especially on the above spaces. Often, however, these 

 spaces are plain, each being bounded inwardly by the scar, gene- 

 rally strong, produced by the post-latero-parietal, and outwardly 

 by the obscurely defined line already mentioned. The two 

 spaces in both valves are occasionally connected at their posterior 

 end by a faint linear scar passing behind the umbonal muscle, 

 and which is evidently due to the attachment of the setal 

 band. Taken together, the posterior part of the band and the 

 two pleurocoeles are represented by a large arch-shaped im- 

 pression, the crown of which is linear and the sides are 

 dilated. 



In fossil Palliobranchs a posterior arch-shaped impression is 

 uncommon. Mr. Davidson and myself are prepared to show 

 that it occurs in the Trimerellids. Doubtless it is present in 

 extinct species oi Lingula. Eecently Mr. James Hall, who has 



