1881.] NATURAL SCIENCES OF PHILADELPHIA. 213 



radial series the first plate is iiiueh the largest, and the number of 

 interradial and anal plates is considerably less in }■ onng specimens 

 than in the adult, thus indicating that the calyx in these crinoids 

 was developed from the basals up, as in their living representa- 

 tives. 



With the development of the first interradial, apparentl}^ simul- 

 taneous with the second and third radials, the Platj'crinoid form, 

 the simplest of the Sphaeroidocrinidoe was complete. The earliest 

 tj'pes of the Platycrinidje known to us, but evidently not the 

 earliest representatives of that group, are from the Upper Silurian. 

 In Coccocrinus the body is composed of three basals, two by five 

 radials — the first very much the larger — a single interradial, and 

 five large oral plates, exactly as we must expect from analogy to 

 find the Actinocrinoid in its earlier phases. The ra3's in the 

 Platycrinoid are free from the primary radials up, but the first 

 joints of the two main divisions are simple and constructed similar 

 to the radials in the hodj of the Actinocrinidae. To transform 

 the Platycrinoid into an Actinocrinoid, it only requires the inter- 

 polation of one or more interradial pieces between the proximal 

 plates of the first division of the ray. By this simple process, the 

 plates which were before free in the Platycrinoid, were incorpo- 

 rated into the body, and raised to the dignity of secondarj^ radials. 



Many of the earlier Rhodocrinidse and Actinocrinidse are char- 

 acterized by highly elevated ridges, which extend all along the 

 radial series of the body. They run verticall}^ along the middle 

 of the primary radials, divide upon the third plate, and branch to 

 the secondary and tertiar}^ radials, whence they pass very gradu- 

 ally into the arms. The ridges are very prominent, rounded 

 exteriorly, and as they approach the arm bases, assume nearl}^ the 

 shape and size of the arms. The plates upon which they are 

 extended, in their upper series, scarcely difier in length from the 

 first free arm plates, and all graduall}^ diminish upward. The 

 longitudinal ridges are evidently not accidental, nor a mere orna- 

 mentation, but represent the arm joints as they were when first 

 developed in the young animal. In this early stage they were 

 round joints, the lateral wing-like extensions being developed 

 afterward, when by reason of the upward growth of interradial 

 and interaxillar}^ pieces, the plates became parts of the body. We 

 find on the surface of many internal casts of forms belonging to this 

 group similar but narrow ridges, which follow the same direction 



