38 VERRILL 



called an " adambulacral jaw," and it is characteristic of the family 

 Asteriidse and of most other families of Forcipulosa. 



External to this proper jaw-plate are a series of two to ten or 

 more pairs of adoral adambulacral plates, each pair closely pressed 

 together (contingent), to form the adoral carina, which may be more 

 or less compressed and thin, and is longest in those species with a 

 large disk, like the Pisasters, in which it often contains ten or eleven 

 pairs of united plates. The spines (adorals) borne on these plates 

 are often longer than those beyond. 



H. ADAMBULACRAL SPINES; ARRANGEMENT. 



Much importance has been given by many writers to the number of 

 adambulacral spines on each of the plates. Many species have regu- 

 larly a single spine on each plate, thus forming a simple regular row 

 (monacanthid) . Others have regularly two spines to a plate, form- 

 ing two rows (diplacanthid) . But still more frequently the spines 

 stand irregularly, one or two to a plate, or alternately one and two, 

 (subdiplacanthid) . In these the spines often appear to form three 

 rows. In Allasterias Ver. the inner spine is situated higher up within 

 the groove, on alternate plates. 



In some cases, as in Pisaster and Coscinasteruis, the monacanthid 

 condition is associated with other important characters, and in these 

 it seems to be really of generic value ; but it should not be used as a 

 character of primary importance, nor by itself, for generic divisions, 

 for in some cases closely related species, or individuals of the same 

 species, may differ in this respect.^ Yet it is a character ordinarily 

 early fixed in the young starfish, and therefore important. 



I. NUMBER OF RAYS; VARIABILITY. 



The number of rays can only be used for subordinate purposes, 

 for it is apt to vary sporadically, even in species that are the most 

 constant, like Asterias vulgaris and A. forbesi of the Atlantic coast, 

 in which we find, when vast numbers are examined, that a certain 

 per cent will have six rays, and a smaller per cent seven rays and 

 four rays, while eight-rayed and even nine-rayed examples may occur 

 very rarely; but those cases with seven and more rays may, in most 

 cases, be due to abnormal repairs after injury. 



In certain species of Leptasterias that carry their eggs and young 

 till they reach the period of bottom life, the young carried by the 



^This is true of Coscinasterias tenuispina and C. acutispina (Stimp.). 



