316 PROCEEDINGS OF THE ACADEMY OF [1879. 



high, and resemble in form and construction the joints of the heavy 



arms 



Arms long, ramifying several times towards the inner part of 

 the ray, the main arms and branches scarcely diminishing in size; 

 in fact the joints near the tips of the arms are almost as large as 

 the brachials. Both are generally short, quadrangular, except the 

 bifurcating plates, which are comparatively longer. Ambulacral 

 furrow deep. Pinnulae wanting. 



Anal plate one, small, resting upon the truncate basal, and sup- 

 porting a lateral tube which is in line with the arm bases. The 

 ventral tube was evidently small. In A. Knappi it apparently 

 rests directly upon the basal, or probably the anal plate forms a 

 part of the ventral sac. The vault is surmounted by five large 

 oral plates all other plates in the dome unknown. 



Column round, of alternately larger and smaller joints. 



Geological Position, etc. Confined to the Upper Silurian and 

 Devonian so far as known, and found both in Europe and America. 



The following species may be referred to this genus: 



I860. Arachnocrinus bulbosus Hall. (Cyatliocr. (?) bulbosus), 15th Rep. N. Y. St. 

 Cab. Nat. Hist., p. 123, pi. 1, figs. 19-22. Upper Helderberg, Devon. Liv- 

 ingston Co., Kentucky. 



*1879. Arachnocr. extensus Wachsm. and Spr. (See description below.') 



*1878. (?) Arachnocr. granulatus Angelin. (Cyatliocr. granulatus), Iconogr. 

 Crin. Suec, p. 23, pi. 16, figs. 1-3 Upper Silur. Gotland, Sweden. 



*1879. Arachnocr. Knappi Wachm. & Spr. (See description below.") 



1 Arachnocrinus extensus, n. sp. The body of the only known specimen 

 is not in a condition to exhibit satisfactorily the arrangement and propor- 

 tions of all the plates, but enough is visible to show that the calyx closely 

 resembles that of Arachnocr. bulbosus. Its plates are smooth, the under- 

 basals small, and projecting but little beyond the column. Arms massive, 

 bifurcating on the second brachial, except in the anterior ray which divides 

 on the 14th or 15th plate, and branches further up four times ; the two 

 lateral rays branch five, the two posterior rays six times. The arms are 

 long and stretched out horizontally to their full length. Arm joints almost 

 as large as the brachials and like them cylindrical, much wider than long 

 except the bifurcating plates which are always comparatively longer, with 

 a thickened rim both at their upper and lower margins. 



This species differs from A. bulbosus in the proportions of its arm joints, 

 from A. Knappi in the number of brachials, and from both in the bifurca- 

 tion of the arms, . Hamilton Gr. below Hydraulic Beds (Encrinal 



limestone, Hall). Devonian, Falls of the Ohio. 



2 Arachnocrinus Knappi, n. sp. Calyx oue and a half wider than high, 

 basal portion concave. Underbasals small, not extending beyond the col- 



