68 VERRILL 



stout and openly reticulated. Dorsal spines few or many, and 

 variously arranged, often in reticulate or acervate patterns, but 

 sometimes scattered or in irregular rows. 



One of the important structural features, characteristic of most 

 of the species, is the increased number of rows of actinal plates and 

 spines, and their close imbricated or tesselated arrangement; but 

 in certain species (P. papulosus') , otherwise related to the type, there 

 are usually only two rows of actinal plates that bear spines below the 

 inferomarginal plates, as in typical Asterias; very large specimens 

 may have more rows. 



But there is another character, perhaps, of more value, common to 

 all the species from the Northwestern American Coast related to 

 P. liitkenii and P. ochraceus, which seems to warrant the separation 

 of this group, as a distinct genus. This is the existence of the 

 peculiar form of large, stout, erect, sessile, dermal pedicellarise, 

 ovoid or wedge-shaped in form, with broad, unguiculate or denticu- 

 lated, interlocking jaws. These occur especially along the naked 

 spaces between the ventral and lateral spines, but they are also 

 frequently found scattered between the dorsal spines, or between 

 the bases of the actinal spines. They are often equal to the adjacent 

 spines in diameter, but are much shorter. More slender, lanceolate 

 or ovate, major pedicellariae, of the ordinary type, always coexist 

 with these large forms, while the latter are not present on starfishes 

 of the Asterias rubens type, nor on those belonging to most of the 

 other divisions. 1 



It is probable, therefore, that this group had its origin upon the 

 Pacific coast of North America, where it has subsequently had time 

 enough to develop into many diverse species, some of which may 

 have migrated to other regions and left descendants. This, then, is 

 one of the most characteristic groups of this fauna, but it is not so 

 strikingly peculiar as Pycnopodia and Dermasterias. 



Mr. A. Agassiz (1877) referred to the name Pisaster as having 

 been proposed by Professor L. Agassiz, for the group of species 

 allied to A. ochracea, but he did not consider the group as more than 

 a subdivision of Asterias, though he fully described the characteristic 

 structures in the skeleton of P. ochraceus, as contrasted with a 

 typical Asterias. Miiller and Troschel had, however, proposed this 



1 A few similar large pedicellarise often occur on Leptastcrias cpichlora 

 (Br.), but not on the other allied species. They are often wanting, and may 

 be due to hybridizing with P. ochraceus, associated with it. Similar large 

 ones are found on Orthasterias columbiana and allied species. 



