INVERTEBRATES. 229 



all the Mesozoic and more modern Echinoids, in having from three to seven or 

 nine rows of interambulacral plates to each series, instead of only two (those of 

 the inner rows being hexagonal, and the two outer pentagonal); and judging 

 from what is known of Melonites, also in having a greater number of pores in 

 each ovarian plate, with likewise sometimes a greater number, and different 

 arrangement, of ambulacral plates and pores. 



Prof. McCoy has, apparently upon sufficient grounds, proposed to divide this 

 group into two sections or families, distinguished by the following characters : 



1. PALJSCH1N1DJE, McCoy. 



With spines small, all of one kind, and articulating upon imperforate tubercles. 

 Includes Meloniles, Oligoporus and Palxchinus. 



2. ARCHjEOCIDARIDJE, McCoy. 



With plates comparatively thin, and spines of two kinds — a smaller secondary- 

 series, and a larger primary series ; all articulating upon perforated tubercles. 

 Includes Archmocidaris and apparently Perischodomus. 



It is possible Lepidechinus, Hall (which seems to be very near Perischodo- 

 mus, McCoy), may be found to typify a third section or family, on account of 

 its peculiar character of having all the plates distinctly imbricating. 



Genus PAL^CHINUS, McCoy, 1844. 



(Carb. Foss. Ireland, p. 171.) 



The species belonging to this genus are usually large, oval 

 or spheroidal, and composed of comparatively thick plates, 

 which are provided with numerous small spines. In the inter- 

 ambulacral areas, there are five to six or eight ranges of plates, 

 those connecting with the ambulacra being pentagonal, and all 

 the others hexagonal, as in other groups of the suborder. The 

 ambulacra are narrow, even, or but slightly furrowed, and each 

 composed of two alternating ranges of small pieces, which are 

 wider than long. Each of these pieces is perforated at the 

 outer end by two small pores, arranged so as to form two 

 double rows, or two single zigzag ranges, to each ambulacrum. 



One species of this genus is said to occur in the Upper Silurian Rocks, and 

 all the others known, have been found in the Carboniferous System. 



