NIAGARA GROUP. 



191 



585. 4. THYSANOCRINUS IMMATURUS («. sp.). 



PL. XLII. Fig. 4 a-/. 



Calyx cup-form or depressed subturbinate ; pelvic plates minute, enclosed within the ex- 

 tended plates of the second series which envelop the upper joints of the column ; second series 

 of plates hexagonal, with a single exception, which is truncated on the upper extremity ; the 

 second series, or costal plates, support on their oblique edges five scapular plates, which 

 alternate with the same number of interscapular plates ; arms dividing upon the second joint 

 above the scapulars. 



This species is known to me only in a single fragment, which, so far as preserved, has all 

 the characteristics of the genus. The first series of plates appear not to be fully developed, and 

 the second series cover them so entirely that they are not visible in a lateral view of the spe- 

 cimen. It is evident, however, from the form of the second series of plates, that they can not 

 properly form the base, or part attached to the column, since they only enclose it. The second 

 series of plates now unite with the first series on their inner edges only, leaving the outer 

 edges projecting. It is possible that this is only an immature state of the species, which, in its 

 fully developed condition, will present five minute triangular or pentagonal pelvic plates, on 

 the oblique upper edges of which the second series of plates will rest. This structure is better 

 shown in fig. 4 f, where the outer circle shows the present extent of the column, while the 

 inner shows the character it would have if developed as just described. 



Fig. 4 a, b. Lateral view of the specimen, natural size and enlarged. 



Fig. 4 c. The upper margin of the broken cup. 



Fig. 4 d. The base of the cup, natural size. 



Fig. 4 e. Figure showing the structure of the cup, and form of the plates as far as the divisions 



of the arms. 

 Fig. 4 /. The base enlarged, showing the relations of the first and second series of plates, 



leaving out of view the overlapping edges of the latter. 



Position and locality. In the lower part of the limestone at Loclqiort. 



Genus MYELOD ACTYLUS {710V. ge7i.). 



[Gr. (aueXo?, medulla, and Saxruko?, digitus.] 

 This generic name is proposed for characterizing the arms or fingers described below, the 

 remarkable feature of which is the foramen or medullary canal penetrating the column of joints, 

 a character not existing in the species where the fingers are composed of a double series of 

 joints, and which is not conspicuous in the simple jointed fingers without tentacula. 



