92 PROF. W. J. DAKIN ON A NEW SPECIES OF 



quite short and columnar-epithelial like. Eventually these become ex- 

 ceedingly small, so that their depth is only a fraction of the thickness of the 

 dorsal wall of the cord. The attainment of this condition marks the point 

 where the wall of the stomochord is continuous with the epithelium of the 

 anterior end of the pharynx (PL 10. tig. 2, St.o.). 



Nuchal Skeleton. 



The nuchal skeleton is well developed in Pt. pelsarti and bears characters 

 which are of diagnostic importance. It consists of cupule^ body, keel, and 

 cornua. 



In Pt. flava, the cupule is figured by Willey as embracing the posterior 

 end of the ventral dilatation of the stomochord. In the Abrolhos species, 

 the anterior margin of the cupule is transversely expanded, and embraces the 

 lateral pouches of the stomochord (Pi. 10. fig. 6, Sk.l.). Proximally to this, 

 the cupule passes into the anterior part of the body. The latter, at this point, 

 is only slightly compressed, and there is no crest projecting into the stomo- 

 chord, although a slight convexity towards this structure may be noted. 



In some species of the Ptychoderidse a ventral keel is present, and at the 

 anterior end this is separated from the body proper by a thin band of 

 chondroid tissue. Willey, describing Pt. flava, states that no keel is present 

 in that species, and that in this respect Pt. flava resembles Pt. erythrcea, 

 where the place of the keel is occupied by the large ventral csecum of the 

 proboscis coelom, against which the body of the skeleton expands. Punnett 

 both describes and figures a keel on certain of his varieties of Pt. flava, 

 without referring to this noteworthy difference from Willey^s description. 

 Either Willey considered that the structure to be discussed below, which 

 resembles the " keel " of Punnett's description, was not really the keel, or 

 else Willey's Pt. flava was considerably different from Punnett's varieties 

 of that species. 



In Pt. ,pelsarti a keel-like ventral ridge is present on the nuchal skeleton. 

 Anteriorly this is separated from the main part of the body by chondroid 

 tissue — but it would, perhaps, be better to say that at this point the compact 

 substance of the skeleton passes into a transverse band of chondroid tissue. 



Slightly posterior to the commencement of the keel, two well-developed 

 wings (PI. 11. fig. 8, Sk.^) appear as lateral projections, and extend outwards on 

 each side for some distance, eventually passing into the basement-membrane 

 which underlies the nerve-layer of the proboscis neck at this point. The 

 figures explain this better perhaps than is possible in a written description. 

 Ventral to the wings of the skeleton the keel is compressed (fig. 8), and 

 hollowed slightly to receive the ventral cascum of the proboscis, which is 

 strongly compressed, as we have seen above. The keel clasps this csecum, 

 its ventral processes gradually thinning out (fig. 8, Sk.'^) and passing into 



