1881.] ■ NATURAL SCIENCES OF PHILADELPHIA. 195 



the arm openings, unless the full length of the ra}' is preserved ; 

 any fracture of the ray, whether cutting off one arm or a dozen, 

 shows in the specimen onl}' a single opening. 



The Platycrinoid with its simple form, is similar in structure 

 to the young Actinocrinoid,in which some portions of the ray are 

 yet in the condition of free appendages. The young Actinocrinoid, 

 at this stage, has the same number of arms as the adult ; but in 

 species with numerous arms, the upper divisions of the ray, which 

 in the mature animal are incorporated in the body walls by means 

 of interradial and interaxillary plates, form free appendages in 

 the younger stage, and consequently the number of arms is 

 comparatively less than in full grown individuals. In Slroto- 

 crini's, which has the greatest number of arms, we find in very 

 3'oung specimens only four arm openings to the raj' ; approaching 

 maturity there are eight, afterwards twelve, and in fully matured 

 individuals there are perhaps sixteen or more. Specimens in 

 different stages of growth, have frequently been described as 

 distinct species on account of variation in the number of arm 

 openings ; and this has even been extended to genera. As growth 

 progressed, the upper branches gradually lost their free character 

 by being absorbed into the body walls through the interpolation 

 of interradials and interaxillaries, both in the calj^x and in the 

 vault. The plates which covered the ventral side of the free 

 appendages were thus drawn into the vault, and became at length 

 a part of the main bodj^ ; but the Platj'crinidfe, which have no 

 interaxillaries nor increase of interradials, retain the free append- 

 ages during life. 



As a general rule it may be asserted, that in the Sphreroido- 

 crinidffi the plates of the vault increase in number outward in a 

 similar manner to the plates of the calyx. The various plates of 

 which the body is composed might be separated into two classes : 



1. Plates which either do not increase in number, or do so only 

 at the distal ends, and not by interpolation, including the basals, 

 radials and arm plates. 



2. Plates which increase by interpolation only, including the 

 underbasals, interradials, anals and interaxillaries, to which we 

 might add the joints of the column. 



