212 THE VOYAGE OF H.M.S. CHALLENGER. 



The Dorsal Lamina is very broad in its posterior half. It is transversely ribbed awl 

 minutely tuberculated at the edge, a larger process being placed opposite the end of each 

 transverse rib, and there are generally a few smaller ones between. 



The Tentacles are thirty-five to forty in number, they are long and touch at their bases ; 

 longer and shorter ones generally alternate, but such is not always the case. 



TIic Dorsal Tubercle is oval or elliptical in outline. The horns almost meet, and are 

 not turned in. The aperture is on the right side of the anterior end. 



This species varies slightly in external form. Most of the specimens are more or less 

 depressed, like those figured (PI. XXXII. figs. 1 and 2), and vary in outline from a circular 

 (fig. 2) to a rudely elliptical (fig. 1) shape. A few specimens, however, are not so much 

 depressed, but rise in an oblong form, or rather as a sort of truncated pyramid, from 

 the base to the branchial and atrial apertures, which in that case may be described as 

 being situated respectively at the ventral and dorsal edges of the anterior end. The 

 expanded margin of the area of attachment is present in all the specimens, and is 

 considerably developed in some (PL XXXII. fig. 1) ; it has in most cases grown around 

 small stones and other foreign objects, which have thus come to be imbedded in the 

 test. 



In size the specimens vary from 3 cm. in length and 2"5 cm. in breadth, to 6'5 cm. 

 in length and 3 "5 cm. in breadth. 



The contrast between the test on the flat, solid, greatly thickened base, and on the 

 thin, flexible, upper portion of the body is very great ; it becomes a little thicker again 

 around the apertures. The vessels in the thickened posterior part of the test are very large. 

 The matrix of the test is hyaline, with small fusiform and stellate cells scattered through 

 it ; bladder-cells are few, spherical, and rather small in the outer part ; a few concretions, 

 generally branched, are also present ; probably they have been formed since death. 



The siphons of the mantle are rather long, and the atrial is much wider than the 

 branchial, which is bent towards the ventral edge in the middle of its length. They are 

 moderately muscular. 



The transverse vessels in the branchial sac are very narrow, and in some places are 

 so encroached upon by the ends of the stigmata above and below as to be reduced 

 to ziozag tubes no thicker than the longitudinal interstigmatic vessels (PI. XXXII. 

 fig. 3, tr.). 



The long curved fang- shaped or tusk-like papillae are a characteristic feature in this 

 sac. They are very large as compared with the size of the meshes, and the ciliation of 

 their convex surfaces is very distinct. The broad horizontal membranes appear to be 

 stretched over the convex side of the papillae, and attached to their apices, thus forming 

 the flattened vesicles seen on the under sides of the papillae. 



The ribs on the dorsal lamina, especially on its lower broad part (PL XXXII. fig. 4), 



