22 



ASTEROIDEA. 



fifteen of these possess a vent, and only three are destitute of that aperture. The 

 24 star-fish which Tiedemann dissected and illustrated with mag- 



nificent plates {Astropecten aurantiacus, Lin.) belongs to the 

 latter group ; whilst Baster's observations were made on Uraster 

 ruhenSy Lin., which is classed with the former, and possesses an 

 anal opening. The vent is sub-central, and lies in general at 

 the left side of the madreporiform plate, sometimes surrounded, 

 as in Oreaster reticulatus, by a circle of small wart-like tuber- 

 cles. The annexed fig. 1 4 shows the size and position of the 

 vent aperture in this large star-fish. 



A portion of the disc, with the vent ^ ° 



in Oreaster reticulatus. 



STRATIGRAPHICAL DISTRIBUTION OP THE FOSSIL ASTERIADtE. 



It was long supposed that the Crinoidece were the only representatives of the Echino- 

 dermata in the Palaeozoic rocks, but recent researches in the Silurian strata of England, 

 Wales, Ireland, and North America, have led to the discovery of Asteriada in several 

 stages of these ancient formations. Professor Sedgwick and Mr. Salter, in 1845, found 

 ^Palaaster obtusus, Forb., in the ash-bed west of Bala Lake, and the same species was 

 afterwards discovered in 1846 by Sir Plenry de la Beche, Captain James, R.E., and 

 Professor Forbes, in the fossiliferous slates of Drumcannon, near Waterford, which are of 

 Bala and Caradoc age, as shown by the fossils of all their fossiliferous portions. The 

 Lower Silurian rocks of North America have likewise yielded Asteriada. Mr. James Hall 

 has described and figured Falceaster Niagarensis, Hall, from the Trenton limestone of the 

 State of New York, and Dr. Billings has described many new forms from the Lower 

 Silurian rocks of Canada, and figured the same in the third decade of Canadian organic 

 remains of the ' Geological Survey of Canada.' 



The Upper Silurian strata of Westmoreland and Shropshire have likewise lately been 

 found to contain some beautiful little sea-stars in fine preservation, and Mr. James Hall 

 has discovered several new forms in the Upper Silurians of the United States. 



The Palaeozoic Asteriada have been studied with great care by the late Professor 

 Forbes, Mr. James Hall, Mr. Salter, and Dr. Billings. All the species belong to extinct 

 genera, which present many singular modifications of structure, and afford interesting 

 points of comparison with some living forms. The following genera have been proposed 

 for the reception of these Palaeozoic Asteriada ; the diagnosis of each is given in the words 

 of their respective authors. 



Genus 1. Pal^easter, Hall. — Arms thick, convex, short, or moderately elongate, 

 and formed of many rows of small, spinous ossicles on the upper surface ; ambulacra deep. 



