PALAEOZOIC FOSSILS. 299 



GENUS AGELACRINUS (Vanuxem). 

 Agelacrinus hamiltonensis. 



PLATE 



Agelacrinus hamiltonensis : Va.nuxem, Geol. Third Dist. New-York, p, 158, f. 80 ; 



p. 306. 1842. 



Body comparatively large, discoid or depressed-convex; the border com- 

 posed of several ranges of imbricating plates, tliose of the marginal 

 range minute, the others gradually increasing in size towards the inner 

 edge of the border. The range of plates adjacent to the inner area or 

 disc is composed of large transversely elongate plates, with alternating 

 smaller ones. 

 Arms long, slender, curving ; the anterior arm and the left antero-lateral 

 and postero-lateral arms sinistral, the other two dextral. The extremity 

 of the right postero-lateral ray extends into the anal area, and passes just 

 behind the ovarian pyramid. The arm-grooves are covered by a large 

 number of elongate triangular plates, arranged along their margins ; 

 those of the opposite sides alternating, and their adjacent ends interlocking 

 with each other. The arms have their origin in a transverse pyramid, 

 situated in the central area about two-fifths of its diameter from the ante- 

 rior margin : this pyramid is composed of six plates, five of them trian- 

 gular, their bases forming the termination of the rays and their apices 

 uniting above ; the sixth or posterior plate, rising from the anal area, is 

 larger and somewhat shield -shaped. The ovarian pyramid is scarcely ele- 

 vated, situated subcentrally in the largest interradial area, composed of 

 nine very elongate triangular plates. Interradial areas composed of com- 

 paratively large polygonal plates ( not squamosely arranged or imbrica- 

 ting ), uniting by their lateral faces, and their centres elevated into 

 angular ridges. The surfaces of the squamose plates which form the bor- 

 der, are simply granulose. 



This species, the type of the genus, diifers from all others yet described, 

 in having two of the rays dextral and three sinistral ; also the plates com- 

 posing the interradial areas are not squamose, as in most other species. The 

 pyramid, originating the arms, is composed of a greater number of plates 

 than any of the Silurian species of the genus. 



The original specimen consists of impressions of the exterior of five or 

 six individuals, some of them quite young, the largest one measuring about 

 one inch and a quarter in diameter. No other specimens of this species have 

 been found, so far as I know. 



Geological formation and locality. In the arenaceous shales of the Hamil- 

 ton group, at Hamilton, Madison county, New-York. 



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