1888.] NATURAL SCIENCES OF PHILADELPHIA. 349 



relations of the ventral plates in Haplocrmus, in view of the discov- 

 ery that it has no central plate? Those plates meet in the center, 

 and cover the mouth substantially in a similar manner as the five 

 orals in Platycrimis ; being, however, more alike in form and size, 

 and more regular in their arrangement. They also closely resemble 

 the five orals of the Pentacrinoid larva of Antedon, but, unlike 

 them, are suturally connected with one another as well as with the 

 radials. The plates also occupy the position of the five interradials 

 of Cyathocrinus and the deltoids of the Blastoidea ; resting like the 

 latter upon the limbs or upper extensions of the radials. 



We have heretofore contended, against the views of Carpenter 

 and others, that the ventral plates of Haplocrimis are interradials 

 and not orals, believing the latter to be represented by the "central 

 plate," which we took to be the homologue of the so-called central 

 plate of Actinocrinidae and Platycrinidae. 



It would seem to follow naturally that with the elimination of the 

 central plate from the question, the chief objections to considering 

 the five summit plates as orals, which impressed us so strongly 

 before, would now be removed. A serious morphological difficulty, 

 however, is still found in the position of the opening which we 

 suppose to be the anus. This, as we have already described, pene- 

 trates the middle portion of one of the vault plates — a structure not 

 found in any other known Crinoid, either in the adult or larval 

 state. The position is the same as that of the anus in the deltoid of 

 the Blastoid genus Orophoc7'inus, which complicates the case still 

 more. 



It is further a fact that in the low^est Silurian Camarata inter- 

 radials ai-e more profusely repi-esented than among Carboniferous 

 forms, frequently extending over the whole ventral surfiice of the 

 calyx, while the orals apparently are unrepresented. From this it 

 would seem to follow that if Haplocrinus represented a larval form 

 of the Palaeocrinoidea, the plates in question could not be orals, or 

 the structure would appear to be at variance with the palaeontolog- 

 ical development of the group. 



For these difficulties we are unable at 2:)resent to offer any ex- 

 planation, but nevertheless we admit that there are very strong rea- 

 sons for regarding those plates as orals. They present a striking- 

 resemblance to the five plates composing the unopened oral pyramid 

 of the Pentacrinoid larva before its separation from the radials 

 by perisome, and there are unquestionably very strong grounds 



