of the Ventriculidae of the Chalk. 285 



may easily escape notice. It is however worthy of remark as an 

 additional contrivance for gaining extent of surface, and an ad- 

 ditional instance of the exhaustless variety of plan which nature 

 adopts in the development of life. 



6. Cephalites retrusus. PI. XIV. fig. 8. 



Plaits broad and very deep : outer plaits 



: inner plaits raised in rather small but very promi- 

 nent projections at regular and close intervals, and in such 

 manner that they range spirally round the whole body, and 

 quincuncially relatively to each other : projections cylindrical, 

 rounding off slightly at the top and with an exactly central 

 and rather deep circular depression (sometimes two) on the 

 top of each projection : wall very thick. 



This form departs from every other which has been named. 

 It is the first and only instance in which we find projections on 

 the inner* plaits, which have been already more than once found, 

 and will be so again, on the outer plaits. The fold which marked 

 the outer plaits of C. bullatus is here found, with striking modi- 

 fications however, on the inside. The projections are much 

 smaller and closer than in that species, but no less prominent ; 

 while each one is again marked by a deep though small and ex- 

 actly central depression. It is altogether a very extraordinary 

 form*. In chalk specimens it would at once be distinguished 

 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 



* Forms like this afford very strong ground of caution against the hasty 

 adoption of any development theories. The whole of the present subject af- 

 fords, indeed, the strongest ground for such caution. We see infinite variety 

 — all subservient to the ends of life ; and throughout which one Unity is 

 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 vertebras, 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. 96) 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. 



