STRATIGRAPHICAL DISTRIBUTION. 



23 



with transverse ossicula, and a single row of adambulacral plates. No disc plates be- 

 tween the rays ; madreporiform tubercle small and single. This genus ranges from the 

 Lower Silurian to the Carboniferous strata. 



Genus 2. Palasterina, iJ/'Coy. — Pentagonal depressed; the arms a little produced, 

 with three or five principal rows of tubercles above, combined with a plated disc which 

 fills up the angles ; ambulacra rather shallow, formed of subquadrate or slightly transverse 

 ossicles, bordered by a single row of large, square-shaped plates, the lowest of which 

 are large and triangular, and support combs of spines. Upper Silurian rocks. 



Genus 3. Stenaster, Billings. — No disc ; rays linear, lanceolate, or petaloid ; ambu- 

 lacral grooves bordered by solid, oblong, or square adambulacral plates ; five pairs of tri- 

 angular oral plates ; two rows of ambulacral pores. Upper surface of the disc and rays 

 covered with small plates, which appear to be tubercular, and not closely fitted together. 

 The generic name is from sienos, narrow, in allusion to the contracted body. Lower 

 Silurian. 



Genus 4i. Petraster, Billings. — This genus has both marginal and adambulacral 

 plates, with a few disc-plates on the ventral side. The general form is deeply stellate, and 

 the rays are long and uniformly tapering. Generic name from petra, a stone. It differs, 

 according to Dr. Billings, from Palasterina by the presence of large marginal plates outside 

 of the disc-plates, and still more from Stenaster, which has neither discal nor marginal 

 plates. It is, however, allied to Astropecten. Lower Silurian. 



Genus 5. PALiEOCOMA, Salter.- — Plat ; all the centre of the disc above membranous, 

 with scattered, star-like calcareous spiculac ; the angles filled up with a similar membrane. 

 Arms formed of several rows of quadrate, reticular ossicles, the external rows fringed with 

 spines. Beneath, the ambulacra are narrow and very shallow, the ossicles square or even 

 elongate, and placed alternately. Two rows of bordering plates, the inner row square and 

 without spines, the outer row oblique and fringed with combs of very long spines ; a 

 loosely reticulated membranous web between the arras. Upper Silurian. 



Genus 6. Protaster, Forbes. — Arms elongate, extending much beyond a circular, 

 closely reticulated disc. Arms formed above of two rows of plates, deeply sculptured and 

 spinous at the edge ; beneath, of two rows of elongate ambulacral ossicles, bordered by a 

 row of large spinous plates. The basal ossicles of the ambulacra, bordering plates, and 

 disc, combined to form a petaloid mouth below. Upper and Lower Silurian rocks. 



Genus 7. Pal^odiscus, Salter. — Arms not produced beyond the large, plated pen- 

 tagonal disc, nor distinguishable from it above ; ambulacra formed beneath of crowded, 



