392 Mr. F. A. Bather on British Fossil Gnnoids : 



Geneeic Diagnosis. 



Cup cyathiform, with plates of medium thickness. IB. 5, 

 B. 5, R. 5. Arms with two main branches, bearing armlets 

 or pinnules. E.' small, oblong or rhomboid, between post. B. 

 and r. post. B. below and x and r. post. R. above, x nearly- 

 same shape as radials. Ventral sac i to | length of arms, 

 composed of primitively hexagonal plates. Stem round or sub- 

 pentagonal, with small pentagonal axial canal and with radial 

 sutures occasionally obscured. 



Description of the Genus. 



The species vary greatly in general appearance. 



Dorsal cufp cyathiform. 



Infrabasals 5 ; pentagonal ; forming a very slight angle 

 with stem ; sometimes a little rounded. 



Basals 5 ; 3 hexagonal, posterior and right posterior hepta- 

 gonal ; sometimes slightly protuberant. 



Eadials 5 ; of normal outline, articular facet occupies from 

 ^ to I width ; the axial canal may or may not be separated 

 from the ventral groove by stereom ^. The radials are more 

 or less rounded and project from the cup, which thus acquires 

 a roughly quinquelobate section. 



Arms vary much in length and appearance, but are never 

 very cumbrous. The armlets or pinnules of the various 

 species, though superticially very different from one another, 

 are closely connected in structure and arrangement. Axial 

 canal for the most part distinct. Covering-plates small, and 

 often irregular and extending over a large part of the ventral 

 surface of the arm. 



Costals from 1 to 6 in each ray, the extreme numbers being 

 exceptional. 



Anal structures : — Eadianal (E') a small oblong or rhom- 

 boid, rests on sloping sides of the |)Osterior and right poste- 

 rior basals, with its upper margins abutting on the right 

 posterior radial and the anal x. 



Anal X or Brachianal an irregular heptagon ; its base, which 

 is more or less horizontal, truncates the upper angle of the 

 posterior basal ; the left side is bounded by the left posterior 

 radial, the right by the radianal and right posterior radial j 

 above it supports in the middle, on a slightly curved surface, 

 a wide plate, and on either side smaller plates. 



* Stereom, any hard calcareous tissue forming skeletal structures in 

 Metazoa Invertebrata and in Protozoa. ' Nature/ xliii. p. 345, Feb. 12, 

 1891. 



