278 paljEontology op newyork. 



CRINOIDEA OF THE HUDSON-RIVER GROUP. 

 Plates LXXVI., LXXVII. & LXXVIII. 



Scarcely more than three well marked species are known in this group, and one of these 

 may be regarded as having existed during the period of the deposition of the Trenton 

 limestone. The fragments of these species, and the columns of the same, appear under so 

 many different forms and aspects as to indicate a greater number of species than really 

 exist, and on this account have been more fully illustrated. The characteristic species of 

 the Trenton limestone ceased to exist with that formation, and the peculiar forms so well 

 preserved in that rock do not appear beyond its termination. At this period the nature of 

 the sediment in its eastern part was not well adapted to the growth or preservation of this 

 class of animals ; and, as might be expected, we find in the western extension of the same, 

 where the mass becomes more calcareous, an increase of the number, and a better pre- 

 servation of their remains. 



In addition to the unequivocal forms of Crinoidea, the western localities of this group 

 have afforded several species of the asteroid form, which have a very close resemblance 

 to the true Asterias. A single fragment of this character has been found in the shales of 

 this group on the south side of the Mohawk valley, but it is too obscure to be described. 

 All the forms here described, as well as those from the Trenton limestone, terminate their 

 existence with this formation, and the next rock in which such remains occur presents us 

 with a new assemblage. 



Genus HETEROCRINUS. 

 [Greek, s-rspoff, irregular, and xpivos, a lily] 



Character. Column more or less pentagonal ; pelvis composed of five plates, which are 

 somewhat irregular ; costal plates in a single or partially double series, pentagonal, 

 heptagonal or quadrangular ; scapular plates regular ; arms variable in character ; fingers 

 composed of a double or single series of quadrangular joints, which are not tentaculated. 



The two species which I have placed under this genus, have a similar pentagonal 

 column, with the pelvic plates similar in each. The succeeding plates are somewhat 

 irregular, but are few in number, and the entire structure very simple, interposed plates 

 being entirely absent. Allhough I have liut two species, and of these only imperfect 

 specimens, it is evident that they may with propriety constitute a distinct genus, on the 

 basis of Miller's arrangement. The third species, which is provisionally placed under 

 this genus, cannot be satisfactorily determined at present. 



