192 



PALEONTOLOGY OF NEW-YORK. 



586. 1. MYELODACTYLUS CONVOLUTUS (n. sp.). 



Pl. XLII. Fig. 5 a, 6; and 6 o- A. 



Fingers'? Composed of a single series of thin joints, which are slightly nodulose or tubercular 

 on the back j ends of the joints somewhat semicircular or crescent-form, with the extremities 

 truncated ; tentacula composed of numerous rounded or slightly nodulose joints, which are 

 attached to the truncated extremities of the finger-joint by a tendon inserted into a perforation 

 in the joint ; fingers usually incurved or convolute ; plates penetrated vertically by an oblong 

 quadrangular canal, through which probably passed a strong tendon connecting the whole 

 together ; surface of each plate marked by a pentagonal depression, within which are elevated 

 ridges, the whole intended for the strong attachment of muscular fibres connecting the plates ; 

 outer edge of the plate, upon the back, marked by two, or sometimes three minute grooves or 

 perforations communicating with the muscular impression upon the upper surface. 



Numerous fragments of this species (like fig. 6 a, b) have been found, composed of a single 

 series of wide flat joints with a semicircular groove on the inner side, showing on the face of 

 the truncated inner angles a minute perforation for the attachment of the tentaculum. These frag- 

 ments are always curved, however slightly, in the same direction, and the specimen fig. 5 a 

 forms more than a single convolution. In a lateral view, there is no visible character to separate 

 it from Thysanocrinus ; but believing that the other characters mentioned are of generic im- 

 portance, it has been separated. 



Fig. 5 a. A fragment rolled together, forming more than a single convolution ; the tentacula 



apparently contracted gradually during the infolding of the arm, and those of the 



outer extremity spread evenly over, enclosing the others. 

 Fig. 5 b. Three joints of the finger enlarged, showing the tentacula to the fifth joint. 

 Fig. 6 a. A fragment of a similar finger, showing the broad groove on the inner side, and the 



minute perforations on the truncated inner angles of the plate. 

 Fig. 6 b. The outer side or back of the same fragment, showing a nodulose surface. 

 Fig. 6 c. Several joints enlarged, showing the character of the inner side. 

 Fig. 6 d, e. The back enlarged, showing in e the minute groove for the passage of a tendon to 



the external surface. 

 Fig. 6 /, g. Articulating surface of the plates, natural size, from two different specimens ; the 



upper one less distinctly defined than the lower one. 

 Fig. 6 h. The same enlarged, showing the form of canal, the pentagonal muscular impression 



with ridges for the attachment of muscles, and the minute grooves extending to the 



back of the column. 



Position and locality. In the shale at Lockport. The fragments are washed out from the 

 decomposing rock by rains. {Collection of Col. Jewett.) 



