MORPHOLOGICAL PAET. 93 



Palaeocrinoidea ; but as soon as it appeared that there is no such plate, it 

 occurred to us that the plate, so apparently central in many Platycrinidae 

 and Actinocrinidae, 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 

 Haplocrimis, and alluded to it in the Eevision, Part III, p. 56, as follows : " K 

 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 Haplocrinusy The last of these diffi- 

 culties which then seemed so serious was met by the elimination of the 

 mythical plate in Haplocrimis ; 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 palaeontological story to us. 



While writing up the observations which w^e had made on Haplocrimis, 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 Ichthyocrinidae had been almost totally unknown. By extraordinary 

 good luck we obtained a specimen of the genus Taxocrimis with the ventral 

 disk in almost perfect preservation, and after carefully cleaning the speci- 

 men, we found that it had an external moidh, surrounded ly five parted oral plates, 

 tvith the ambulacra converging to it, and passing in hetween the orals. (Plate III., 

 Fig. 11.)* 



The middle of the disk is occupied by fivQ 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 Struc- 

 ture of Taxocrinus and Haplocrinus, and Consequent Modifications in the Classification of the Crinoidea." 

 Proceed. Acad. Nat. Sci. Phila., Nov. 27, 1888. 



