V<d T. 



Grabau — Ordovician Fossils from North Chinn 



(I) 59 



air-chamber, first by effecting the elongation of the outer part, that which forms the new 

 shell built by the mantle edge, and then, by the subsequent withdrawal, for a space, of 

 the base of the reflexed part of the mantle, effecting the building of the basal layer of 

 lime, -which is the so-called septum of the camerate portion of the Holochoanites. 



Fig. 17. Hypothetical restoration of a primitive holochoaiiitic cephalopod, represented as resting with its 

 ventral or hypononiic side upon the sea-floor, and with the shell sectioned. The stage here represented is at the 

 beginning or the building of the camerw, previous to which it consisted only of the precamerate portion of the 

 " siphuncle". This is here represented in section with several conical endosheaths, the spaces between which are filled 

 with solid stereoplasm, except the median tube or " endosipho-tube ", which ends in the embryonic bulb, this being 

 however non-calcareous and not preserved. The animal rests upon the final endosheath of the stage which surrounds the 

 I'liilocoiic of this period. The hypothesis of camera-building by a reflexed fold of the mantle, analogous to the dorsal fold 

 of the mantle in Xniililas, is here illustrated, the beginning ot the first camera on the dorsal side being shown. 

 ANIMAL: a, mantle; b, marginal reflexed fold of the same, which is assumed to be functional in camera building; c, 

 shell-muscle; d, hood; e, sipho, occupying the "endosipho-tube";/, embryonic bulb, non calcareous (when calcifield it 

 forms the protoconch); g, hyponome; h, tentacles or arms; /, eye (left side), SHELL: ./, shell of early stage ('. e. of 

 preseptal part of "siphuncle", (ectosiphuncle of Ruedemann) ; /, continuation of same into camerate state at the 

 contraction of the "siphuncle", forming the " endosipholining " of authors; k, last-formed endosheath at this stage, 

 enclosing the " endocone " which is continued in the "endosipho-tube " ; II -13, earlier sheaths (septa of primitive shell) ; 

 ml - lui, " siphon al " chambers (camera' of primitive shell) filled solidly with stereoplasm; //, new shell or shell of 

 camerate portion; o, shell-lining of first camera, deposited next to the continuation of the "old shell" (/. e. the 

 continuation of shell of "siphuncle", the so-called endosipholining) and forming on the hypothesis here suggested the 

 "niphonal funnel " or " neck ' ' of the first " septum " which has not yet been built. 



I am perfectly well aware that this interpretation meets with a grave objection 

 because of the fact that in Endoceras, Vaginoceras etc., the septal portion of the camera 

 is continued downivards in the siphonal neck, not upwards as such a mode of construction 

 would require. But it must be remembered that in these forms, the septal necks take the 

 place of the inner shell or the endosipholining, which is absent in these genera. 

 Whitfield has recorded the observation, that the septal necks of Vaginoceras are continu- 

 ous with the sheaths of the siphuncle though Ruedemann holds that this needs 

 verification. If it is correct, then the septa merely mark the rapid outward expansion of 

 the mantle above the edge of the endosheath (which is really the upper edge of the 

 siphonal neck) there being no further need for a reflex of the mantle on the suppression 

 of the inner shell. 



That such a reflexing of the mantle has occurred in some of these ancient 

 cephalopods, is shown by the structure of Orthoceras truncatum Barrande, from the 

 Ordovician and Silurian rocks of Bohemia and England. In this form the earlier camerte 



