126 Dr. H. A. Nicholson on Species 
small circular tubes, rendering it minutely porous (Pl. XI. 
fig. 3d). 
The examples of A. frondosa which have come under my 
observation are most commonly attached to the valves of 
Strophomena alternata, S. planumbona, and Orthis occidentalis ; 
but I have also seen the crusts growing on Chetetes fron- 
dosus and C. mammulatus. 
Locality and Formation —Hudson-River Group (Lower 
Silurian), Cmeimnati and Waynesville, Ohio. The specimens 
figured are reticulated examples, and are not so characteristic 
as the expanded and confluent forms. 
4. Alecto confusa, Nicholson. Pl. XI. fig. 4. 
Polyzoary adnate, forming thin crusts which envelop foreign 
bodies. Cells tubular, multiserial, arranged in irregular trans- 
verse rows, Immersed towards their bases, free and elevated 
above the surface towards their apertures. Cell-mouths cir- 
cular, terminal, as large as the diameter of the tube, about 
five in the space of one line. 
All the examples of this species that I have seen are para- 
sitic upon the columns of Crinoids, which they closely embrace 
and incrust ; and they might readily be referred to Aulopora, 
unless care were exercised. The species is nearly allied in 
essentials to the two preceding, but is distinguished by its 
constantly forming thin crusts, and by its larger, more closely 
and irregularly arranged, and more prominent cells. 
Locality and Formation.—Cincinnati Group, Cincinnati, 
Ohio. Collected by Mr. U. P. James. 
5. Aulopora arachnoidea, Hall. Pl. XI. figs. 5, 5a. 
Aulopora arachnoidea, Hall, Pal. N. Y. vol. i. p. 76, pl. xxvi. figs. 6 a-6 ce. 
Corallum very slender and delicate, attached to the surface 
of foreign bodies, repeatedly branching, and in many examples 
anastomosing to form a network. The branches are usually 
given off at intervals of from one third to two thirds of a line, 
and are very narrow and linear, not exceeding one fifth of a 
line in width. The corallites have much the form and cha- 
racter of the cells of the uniserial forms of Alecto, being in- 
variably arranged in single lines and opening in the axis of 
the branches. The terminal portion of the corallite is elevated 
above the surface; and the calices are circular and not ex- 
ped About four or five calices occupy the space of one 
ine. 
