LOWER HELDERBERG ROCKS. 139 



Mariacribils : Subgeims Technocriiins. 



In some species of Mariacrinus I have noticed a modification of the 

 arm-structure, which is manifested only above the third series of brachial 

 plates, and which affects the parts above that point ; giving gi-eat difference 

 in the structure of the arms of these species, as compared with the three 

 first described, M. nohilissimus, M. pachydactylus, and M. paucidadylus. 

 In the species of the Oriskany sandstone we have the essential structure 

 of Mariacrinus, as far as the comniencement of the brachial plates. Above 

 this point, instead of a series of three brachial plates resting on the third 

 radial (as in Plate ill, fig. 4 & 10), below the first bifurcation, we have in 

 these forms the bifurcation occurring directly above the first arm-plates, 

 which thus give origin to two pairs or four arms from each ray. The in- 

 terbrachial plates are also wanting in these species. The arms are simple, 

 or composed of alternating ranges of plates. 



The subgenus may be characterized as follows : 



Teciiwocrwxs. 



Basal or pelvic plates four. Radial or costal plates three in series of five 

 (3X5). Interradial or intercostal plates four or five. First brachial or 

 scapular plates two resting on the upper sloping edges of each third 

 radial (2x5 = 10), which are directly succeeded each by two other 

 plates, or second brachial plates, twenty in number. A second series of 

 similar plates, succeeded by the smaller plates of the arm, which may 

 consist of a single or double series. 



Although preferring to characterize the genera of Crinoids from the 

 fundamental structure of the body, leaving those parts above the brachial 

 plates to determine specific distinctions, there seems in this instance some 

 reason for making a subgenus of tho.se forms which present such degrees 

 of variation from the originaj type as do these species of the Oriskany 

 sandstone. The first deviation is noticed in the M. plumosus, which, though 

 having essentially the same structure at the base of the arms, has never- 



