1881.] natural sciences of philadelphia. 181 



1. Underbasals and Basals. 



The genus Glyptocrinus Hall, from the Lower Silurian is one 

 of the earliest, most beautiful, and most instructive types of the 

 Pala?ocrinoidea, and fortunately^ is often found in excellent preser- 

 vation. Looking at the great number of plates which compose 

 its body, at its elaborate ornamentation, one is naturally inclined 

 to consider this as one of the most maturely developed forms in 

 the whole family, but in other respects, it possesses in a marked 

 degree the characters of the young crinoid of later geologic 

 times. 



Glyptocririus was originally described with five basals and no 

 underbasals. Hall afterwards discovered in Gl. decadactylus 

 small pieces concealed within the basal cavity, so rudimentar}-, 

 however, that both he and Meek hesitated to call them basals, 

 although both authors apply that term to the proximal plates in 

 all other cases. Meek distinguished them as " subbasals." We 

 have examined the plates in question very carefnll}" in the species 

 named, and find them, although very rudimentary, placed within 

 the basal ring, hence they are, according to our terminology, true 

 underbasals, and not as Hall describes them a " quinquepartite " 

 upper joint of the column.^ 



In some other species of this genus the underbasals seem to be 

 altogether wanting, at least are not developed externalh'. In 

 Glyptocrinus Dyeri no trace of them can be discovered, thovigh 

 we have examined with reference to this point, some most perfect 

 specimens. If the underbasals were elements of famil}^ import- 

 ance, Gl. decadactylus and Gl. Dyeri would be representatives 

 of distinct families. 



Glyptocrinus is exclusively a Lower Silurian genus. The two 

 species from the Upper Silurian, referred to it by Hall, have been 

 transferred by us to other genera. One of these, 3Iariacrinus 

 Carleyi, is another interesting case illustrating our view that the 

 underbasals have no important effect upon the general structure 

 of the body. M. Carleyi would be an excellent Glyptocrinus 

 were it not that the cal^'x below the radials is composed of a single 



^ The underbasals cannot be developed from a columnar joint, or their 

 sutures would correspond with the sutures of the column ; whenever this 

 is divided, the division occurs alternately with that of the underbasals, and 

 as a rule alternately with the proximal ring of plates. 



