MORPHOLOGICAL PART. 93 
Palzeocrinoidea ; but as soon as it appeared that there is no such plate, it 
occurred to us that the plate, so apparently central in many Platycrinidie 
and Actinocrinide, might be a posterior oral, pushed inward to a central 
position by anal structures. This interpretation seemed to us one of the 
greatest force, more likely than any other to answer the conditions of a valid 
homology, and to remove the principal objections that had been brought 
forward by Carpenter and ourselves respectively to other theories. 
The idea of referring the plate to the orals was not altogether new. We 
had already taken it into consideration before we knew the real structure of 
Haplocrinus, and alluded to it in the Revision, Part III, p. 56, as follows: “A 
far less objectionable interpretation of the central plate than that given by 
Carpenter would be to regard it as a posterior oral. In this case the orals 
would be represented by five plates, and not by six; the anus would be 
placed outside the oral ring, and the radial dome plates would occupy the 
same position towards the orals as the calyx radials toward the basals. But 
it would place the mouth underneath the posterior oral, and it offers no 
explanation of the central piece in LHaploerinus.” The last of these diffi- 
culties which then seemed so serious was met by the elimination of the 
mythical plate in /Zaplocrinus ; and the first was destined to be perfectly 
cleared up by the recovery of a new fragment from the scattered pages by 
which Nature unfolds her palxontological story to us. 
While writing up the observations which we had made on Haplocrinus, we 
made another still more unexpected and striking discovery, which in our 
opinion settled the oral question in conformity with the last mentioned sug- 
gestion beyond all controversy. Up to that time the ventral structure of 
the Ichthyocrinide had been almost totally unknown. By extraordinary 
good luck we obtained a specimen of the genus Taxocrinus with the ventral 
disk in almost perfect preservation, and after carefully cleaning the speci- 
men, we found that it had an external mouth, surrounded by five parted oral plates, 
with the ambulacra converging to it, and passing in between the orals. (Plate III., 
Figs 1t:)\* 
The middle of the disk is occupied by five rounded or very obtusely 
polygonal plates, interradially disposed, rather oval in outline. The two 
antero-lateral plates are tolerably good-sized, and the postero-lateral ones 
slightly smaller. The posterior plate is nearly three times as large as any 
* A full account of this unique specimen was given by us in a paper, “‘ Discovery of the Ventral Strue- 
ture of Zaroerinus and Haplocrinus, and Consequent Modifications in the Classification of the Crinoidea.” 
Proceed. Acad. Nat. Sci. Phila., Nov. 27, 1888. 
