292 PROCEEDINGS OF THE ACADEMY OF [1879. 



totally at variance with the elemental 1 }* plan of structure in the 

 family, as the calyx here appears to be composed of more than five 

 radial plates, some of the rays having apparently two or even more. 

 Looking at Diagram PI. 16, Fig. 2, showing the arrangement of 

 plates in H. simplex Hall, it will be found that in the anterior 

 and right posterior rays, there are three plates in succession, 

 whereas there are four in each of the other rays. But it will be 

 also observed that the first radial plate in the former is about 

 equal in size to the first two plates combined in the three other 

 rays. It is, therefore, not unreasonable to suppose that the two 

 here represent a compound plate, and are homologous with the 

 single radial in other Cyathocrinidae. This interpretation fully* 

 restores the family* relations, the two succeeding plates being con- 

 sidered as true brachials. A similar construction exists in Den- 

 drocrinus, but there only the left posterior radial is compound. 



The absence of underbasals in some of the species is a good 

 illustration of our view that the underbasals do not constitute 

 principal elements in the structure of the Paloeocrinoidea, but 

 are merely the result of growth and development in geological 

 time. Even Meek, who calls the proximal plates in all other 

 genera of the Cyathocrinida? basals, substitutes for those of 

 Heterocrinus the name "subbasals." 



Generic Diagnosis. General form elongate and slender. Calyx 

 small, subcylindrical, tapering but slightly from the column 

 upward. 



Underbasals minute, in some species almost undeveloped, and 

 appearing externally as subtrigonal points at the lower ends of 

 the sutures between the basals; in some species apparently 

 w*anting entirely. Basals five, subequal, pentangular. Radials 

 irregular; some of the rays differing from those of other genera 

 in having compound instead of simple plates, which are divided 

 by horizontal sutures; upper articulating margin straight. The 

 radials are succeeded by two to four brachials, quadrangular, the 

 upper one a bifurcating plate and supporting the arms. 



Arms comparatively long, simple or branching, composed of 

 single joints with almost parallel suture. Pinnulae heavy, springing 

 alternately* from every second or third arm plate. 



Anals not supported by the basals, but resting upon the upper' 

 sloping margins of the adjoining radials. They consist of a single 

 row of plates, longitudinally arranged, the outer side rounded 



