of the Chalk. 189 



their places upon the more elevated parts seen in the figure. If 

 the reader will refer either to an actual specimen of Halodactylus 

 diaphanus or hirsutus, or to the figures of the former in Dr. Farre's 

 paper, pi. 25. figs. 2 and 3, he will perceive that the actual place 

 of protrusion of the polyp itself is not more plainly seen in these 

 recent specimens than in the specimen which I am describing; 

 and the peculiar mode of drawing-in the body which charac- 

 terizes this class at once explains the reason of this apparent ob- 

 literation of the cell's mouth. I apprehend that the preservation 

 of this fragment is owing to the circumstance of its having been 

 suddenly torn from a living animal, the portion of polypidom 

 attached to it having lost its soft contents in, or soon after, the 

 rupture, and the torn fragment having thus become hermetically 

 sealed up, according to the process already described, in solid 

 flint. Such is the actual condition in which it is found, and in 

 no other condition or manner can I conceive it probable that any 

 of the polyp-skin itself would be preserved in flint*. 



It will be understood from what has preceded, that in the 

 Ventriculidae the entire exposed surface, external as well as in- 

 ternal, except the roots and the heads of the head-bearing species, 

 were covered with polyps. 



Another point remains to be noticed in which the Ventriculidse 

 are found to agree with the Halodactylus hirsutus. 



In describing the Alcyonidium hirsutum [Halodactylus hirsutus) , 

 Dr. Johnston (p. 361) characterizes it as "marked with numerous 

 yellowish circular spots irregularly disposed f ' and informs us that 

 " these yellowish spots are produced by clusters of ova lying im- 

 bedded in the cellular texture" The ovarian cells of the Ventri- 

 culidae might best be briefly described in exactly the same terms ; 

 and this mode of development of the ova is another illustration 

 of the singleness and entirety of the whole polyp-mass f. The 

 ovarian cells in the Ventriculidae lie imbedded in the substance 

 of the polypidom, replacing, where they occur, the substance of 



* It appears obvious that from the very nature of the epidermis it would 

 never be likely to be hermetically sealed up like an isolated fibre — since, 

 though inclosed, there would be communication between its parts, which, 

 over such a continued extent of surface, would not be likely to be entirely 

 destroyed. Hence, like fibre which preserved its connexion, it would in 

 time decay away, and either leave a hollow or be subsequently filled up by 

 calcedony. See ante, p. 85. The epidermis would readily separate after 

 death, but any fragments of it in this state would form but an indistin- 

 guishable mass whether preserved in chalk or flint. 



t I had prepared an extensive series of notes illustrative of the general 

 analogy between the embryology of the Ventriculidae and that of many 

 recent zoophytes, but I have not embodied them above, fearing that it 

 would tend to complicate the subject and distract attention from the con- 

 nected argument which I have endeavoured to set forth demonstrative of the 

 affinities of the Ventriculidae. 



