C2 DR J. H. ASHWORTH 



its posterior margin, so that in its origin it is similar to that of Lingula (see p. 49). 

 The broad base of the elevation is ventral, the posterior or dorsal side of the ridge 

 lies parallel and close to the dorsal mantle-fold,* and the anterior side is almost 

 parallel to the posterior wall of the body proper. The epithelium covering the 

 peduncle is continuous with, but much thicker than, the inner epithelium 

 of the ventral mantle, which unites with the epithelium of the body-wall proper 

 just anterior to the front margin of the peduncle. The central portion of the 

 peduncle consists of strong curved muscle-fibres, which have a general ventro-dorsal 

 direction. A narrow transverse cavity is present in the posterior portion of the 

 peduncle, but this does not appear to have a defin'te epithelial lining, nor is there 

 any visible connection in the preserved specimens between this cavity and the body- 

 cavity, though there is probably a connection in life, as in the case of Lingula (p. 50 

 and fig. 6). The narrow strip of mantle immediately posterior to the peduncle is 

 strongly pigmented, and the pigment extends forwards right and left into the 

 epithelium which covers the sides of the peduncle. The dorsal and anterior faces of 

 the peduncle are almost completely covered with a thin film of periostracum. The 

 peduncle is in condition to be extended ready for fixation ; the small lip at the 

 posterior margin of the ventral valve, which consists only of periostracum, would be 

 bent ventrally, and the neighbouring region of the ventral valve would yield to some 

 extent so as to permit the exit of the peduncle between the shell-valves. During 

 extension the peduncle would be so bent as to bring its flat, previously dorsal, 

 surface into contact with the object to which fixation was about to take place, and 

 the granular cells forming the thick epithelium of that face of the peduncle would 

 secrete the necessary cementing substance. 



The median tentacle is an elevation on the epistome about 30-40/x, in height and 

 50/x in width. Its apex is traversed by a shallow median groove, the cell of which 

 are pigmented. The apical portion of the tentacle is solid, but the basal half 

 contains a narrow axial lumen, which opens proximally into the arm-sinus. The 

 lumen is lined by cells and contains longitudinal muscle-fibrils by means of which 

 the tentacle can be contracted or bent laterally or ventrally towards the mouth. 

 Supporting tissue, like that found in the tentacle of Lingula (see p. 51), is not 

 present, but there is a nervous layer between the lumen and the epidermis. The 

 tentacle is ciliated along two tracts (indicated by the darker areas in fig. 11), one on 

 each side, near its base. 



The discoidal lophophore bears the mouth near the middle of its ventral surface, 

 and on each of its lateral margins there are, in all the specimens except one, four 

 approximately equal cirri arranged as shown in fig 11. In one of the larvae (not 

 the smallest) there arc four cirri, all of normal size, on the right side, but on the 

 left there are only three, the fourth or most anterior cirrus being absent. The cirri 



* The peduncle is not in any way connected with the dorsal valve ; its posterior surface may touch the dorsal 

 mantle-fold, hut remains quite indejjendent of it. 



