32 • PROF. E. W. CLAYPOLE ON HELICOPOEA. 



calcareous expansion, usually fan-shaped ; interstices round, 

 having on one side from three to five rows of cell-openings, 

 the margins usually not projecting ; interstices connected by 

 thin, transverse, non-poriferous dissepiments ; reverse rounded, 

 striated or granulated." (Synopsis of Carb. Foss. p. 206.) 

 1857. Lyropora, Hall. " Bryozoum consisting of foliate reticulated 

 expansions, margined on either side by strong stony supports, 

 which diverge from the base, curving outwards and upwards. 

 The foliate expansion is spread out between these diverging 

 arms, which are themselves formed by the coalescing and 

 thickening of the branches." (Proc. Am. Assoc. Adv. Sci.) 



It is scarcely necessary to add to this list Ichthyorhachis and 

 Glauconome, M'Coy, which are not fenestrate species, having no dis- 

 sepiments *. 



There may be difference of opinion concerning the value of these 

 groups, but most of them are now regarded by American geologists 

 as genera. The discovery of species yet unknown may in the future 

 unite one or more of them with others ; but for the present their 

 limitations are sufficiently distinct for palseontological purposes. 



The three fossils which form the subject of this paper show cha- 

 racters which place them in strictness outside of all the genera 

 above enumerated. Further considerations on this point will 

 follow presently. I therefore propose to place them by themselves 

 in a new genus of Fenestellids, defined for the purpose as follows : — 



Helicopoea, n. g. 



Polyzoary expanded, fenestrate, and spiral, formed of slender 

 bifurcating rays, poriferous on one face, connected by non-poriferous 

 bars, forming an open network ; cells arranged in two rows along 

 the rays, one row on each side of a median keel. Axis none, or 

 consisting only of the thickened inner border of the polyzoary, not 

 straight, but forming a spiral rounded non-poriferous or slightly 

 poriferous inner margin. 



Helicopoea latispiealis, n. sp. Plate IY. figs. 1, la. 



Sp. char. Polyzoary very widely expanded, sometimes as much as 

 eight inches in diameter, very flat, curving downwards towards 

 the centre as into a funnel. Whorls about half an inch apart, 

 dextral or sinistral. Rays about fifty (40-50) in an inch ; fenestrules 

 about twenty-five (22-28) in an inch in length, and from forty to 

 fifty in an inch in breadth, according to the number of rays. Cen- 

 tral axis or shaft very thin or entirely absent, and indicated only 

 by a very slight thickening of the inner margin of the polyzoary. 

 Outer or lower surface poriferous. Rays keeled. 



* It may be worth while to point out here the misuse of the word " inter- 

 stice " in many descriptions of Polyzoa. This term properly signified a space 

 or interval, and not its boundary. It should consequently, if used at all, be 

 applied to the fenestrules, and neither to the rays nor the bars (dissepiments). 

 It would be as well to avoid the use of the term altogether. 



