70 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 14, 



but the apertures of the cavities maintained throughout the same 

 character and relative surface-position or amount of protrusion, due 

 allowance being made for the state of preservation. This description 

 of increase deserves attention, as one means towards forming a correct 

 determination of the class to which an obscure fossil may belong. In 

 certain Bryozoa, as Hormera, a great downward thickening also oc- 

 curs ; but the visceral cavities, which at the upper extremity of a 

 branch or main stem project considerably, are progressively immerged 

 by external additions to the surrounding interspaces ; and this surface- 

 change depends on the viscera occupying permanently the same situa- 

 tion in the cavity, which, being once perfected, is not afterwards 

 lengthened. On the contrary, among Anthozoa of similar branched 

 modes of growth, the digestive organs occupy successively a higher 

 position, forming generally below them a thin plate or a complicated 

 structure ; and they are thus enabled to maintain constantly the 

 same relation with respect to the general surface, the cavity being 

 extended proportionably. Allusion has been already made more than 

 once to the existence of transverse plates in Choristopetalmn impar ; 

 and in the character of the abdominal apertures is an additional con- 

 firmation of the fossil being a true Anthozoon ; while Heteropora has 

 been placed by general consent among the Bryozoa. 



Respecting the parasitical mode of development, satisfactory evi- 

 dence was afforded by a translucent slice of a specimen which had 

 surrounded a well-defined apparently testaceous tube : that the en- 

 veloped body was not the sheath of a perforating mollusk, was evident 

 from the characters about to be noticed. Immediately on the tube 

 was an irregular layer of an indistinct nature, but plainly the base of 

 the coral ; and upon it rested, in some places, more or less conform- 

 ably, abdominal cavities, which, after a limited but variable range, 

 inclined upwards or outwards : occasionally the hollows assumed al- 

 most at once a position vertical to the base ; and a few, distinct cir- 

 cles marked, it was conceived, transverse sections of tubes, which had 

 taken a contrary line of growth : in one part also a decided fasciculus 

 or branch-like group sprung from the base, but it did not extend to 

 the surface, being overlaid by a confused aggregate of structures. 

 The coral zone throughout its whole circuit and breadth abounded 

 with indications of disturbed growth, with unconformable extensions 

 over portions which had been accidentally destroyed : in some places 

 likewise the abdominal cavities had a zigzag form ; the whole amount 

 of irregularities clearly indicating that the polypes had founded their 

 edifice upon an unstable basis. Had this section been examined by 

 itself, the coral might have been considered as an Ascidian zoophyte 

 referable to the family Tubuliporidae*. Many however apparently 

 obscure signs of structure, and which might have been disregarded if 

 seen alone, became with the aid of more satisfactory evidence im- 

 portant proofs of the fossil's true nature, and supported a different 

 conclusion. In the best-developed abdominal cavities a transverse 



* Consult M. Milne-Edwards's enlarged figure of Tubulipora verrucarla, Ann. 

 Sc. Nat., 2nd Ser., Zool., tome viii. pi. 12. fig. 1 b, 1838; or Recherches sur les 

 Polypes, &c. 



