34 VERRILL 



C. FORM AND ARRANGEMENT OF THE DORSAL OSSICXES. 



The form and arrangement of the dorsal ossicles vary considerably 

 and often afford characters of much value. Aside from the stichas- 

 terial arrangement, there are two main types, — the reticulate and 

 the longitudinal, — in both of v^hich the principal plates are lobed, 

 substellate, or cruciform. The former, which is seen in true 

 Asterias, Pisaster, etc., includes several subtypes, — as the stellate- 

 reticulate, seen in Pisaster Ussipinus; the areolate, seen in E. tro- 

 schelii; and the irregular or indeterminate, seen in Asterias vulgaris, 

 A. rubens, etc. These variations depend partly upon variations in 

 the forms of the plates and partly on their arrangement and the 

 existence of supplementary ossicles, and especially on the size and the 

 form of the papular areas. 



The abactinal skeleton of the longitudinal type may consist of 

 only three rows of stout dorsal plates, besides the marginals, with 

 few or no supplementary ossicles, as in Coscinasterias; or the 

 median may alone remain distinct, as in some species of Urasterias. 

 But in other related groups there may be five or more regular rows. 

 These last have but three rows when quite young, so that the forms 

 with three rows are probably more primitive. Urasterias may be 

 still more primitive. Its skeleton is very likely a degenerate type. 

 In the stichasterial arrangement the plates become broader, more 

 angular, and less lobed, and they are usually united directly together 

 by their overlapping or adjacent edges in rather regular, longitu- 

 dinal, imbricated rows; or they may have a tesselated arrangement. 

 The intervening papular areas are consequently small and rather 

 regularly arranged. 



D. DORSAL spines; FORM AND ARRANGEMENT. 



The number, form, and arrangement of the dorsal spines vary 

 widely. Their arrangement, when coincident with that of the plates, 

 is a matter of much importance, but widely different appearances 

 may occur in a single species, due to the variable number of spines 

 that may occur on a single plate, combined with their variations in 

 size and shape. No character is more likely to lead to mistakes in the 

 determination of species and genera than the appearance of the 

 dorsal spines, especially in those groups which, like restricted 

 Asterias, have very numerous spines with an indeterminate or 

 irregular arrangement. But in forms that have a limited number or 

 regular arrangement of plates, the spines often conform strictly to 



