LOWER IIELDERBERG ROCKS. 
139 
Mariacrinus : Subgenns Teclinocriiiiis. 
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 great difference 
in the structure of the arms of these species, as compared with the three 
first described, M. nobilissimus, M. pachydadylus, and M. paucidadylus. 
In the species of the Oriskany sandstone we have the essential structure 
of Mariacrinus, as far as the commencement of the brachial plates. Above 
this point, instead of a series of three brachial plates resting on the third 
radial (as in Plate hi, 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 : 
Teciinocrinus. 
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 ( 2 x 5 = 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 those forms which present such degrees 
of variation from the original 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- 
