78 



from every other species by presenting, on its inner surface, the 

 appearance of a series of small rings, quite unconnected with each 

 other, but arranged with the utmost regularity. 



It is an extremely rare species. I have only met with a single 

 specimen, and that is a cast of the inner surface in flint, with frag- 

 ments of the characteristic ventriculitic structure preserved in 

 places. I am unable therefore to describe the outer plaits. The 

 characters of the inner ones are, however, so marked that those 

 of the outer ones are quite unnecessary in order to establish the 

 specific difference. 



The name retrusus may be considered either to express the ex- 

 traordinary degree in which the inner plaits are drawn back to 

 form the projections; or that the most marked characters of the 

 species are hidden from external observation by being on the in- 

 ner plaits. In either sense the name seems equally appropriate. 



7. Cephalites catenifer. PL XIV. figs. 9, 14, 15, 16. 



Plaits broad and deep : outer plaits projecting prominently at 

 irregular intervals; projections horse-shoe shaped, with one 

 arm of a lower projection often linked to the hoop of the pro- 

 jection above it on the same plait ; occasional points of ana- 

 stomosis between adjoining plaits : inner plaits having large 

 and generally oval depressions at regular intervals ; bulging 

 on each side around depressions till adjoining plaits meet and 

 open into each other : processes very conspicuous : wall very 

 thick. 



Var. Annulatus. 



Plaits broad and deep : outer plaits projecting prominently at irre- 

 gular intervals; projections ring-shaped, and generally running 

 into each other on the same plait and often anastomosing with 

 those on adjoining plaits so as to form connected rings over the 

 whole surface : inner plaits having large and generally oval 

 depressions at regular intervals ; bulging on each side around 



traceable ; but a Unity which certainly no more points to a low type of orga- 

 nization, or to a necessary or probable progressive development of one form 

 from another, than does the beautiful and philosophical demonstration of the 

 cranial vertebra, or the fact of that demonstration being afforded by the 

 most different members of the Vertebrata. It should be noticed that the 

 very remarkable octahedral structure already developed as characteristic of 

 the membrane of the Ventriculidae has no relation whatever to those "geo- 

 metrical figures " alluded to by Professor Owen. In the present case it 

 is a relative, and not a positive, form ; and one assumed by animal fibre for 

 a special purpose. It has been already remarked (p. 24) that no spicules, 

 or " calcifying salts " enter into the composition of any of the Ventriculidae. 

 See Owen "on the Archetype and Homologies of the Vertebrate Skele- 

 ton," 1848, p. 171. 



