32S ijESCRiPTioN OP [Polygastncd. 



four ; centre of disc smooth or minutely punctate. Average diameter 

 1 -230th. A fresh water species, living and fossil in Great Britain 

 and Ireland. 



It aj^proaches C. radiosus, (Ehi\); but its cortas are more numerous 

 and longer. 



It may possibly be identical with C. noricus, (Ehr.) 



Campylodiscus spiralis. = Surirella spiralis, (Kiitz.) — Valves 

 elliptical ; frustules twisted so as to present a spiral outline ; costae 

 distinct, about sixty, parallel or slightly radiate ; centre smooth or 

 minutely punctate. Average length l-170th; breadth l-400th. 

 In fresh water, Guildford, Surrey. 



C. crihrosus. — Valves orbicular; disc marked with radiating lines 

 of minute perforations, crowded towards the margin. Diameter 

 l-240th. This probably = C. JEcheneis, (Ehr.), but the description 

 of the central area in that species as solid, i. e. unperforated, will 

 not apply to the present ; the perforations extending over tlie entii'e 

 surface, though, more distant, and somewbat scattered in the middle. 



C. parvulus, (Smith.) — Valves orbicular ; disc traversed by two 

 parallel ridges ; striae about twelve, nearly parallel. Average 

 diameter l-550th. Poole Bay, 1848. This species is readily dis- 

 tinguished by its minute size;, and the ridges on its valves, which 

 are very prominent in certain portions of the frustule. It does not 

 appear to have been noticed, either by K tzing or Ehrenberg. 

 (P. 24. f. 22, 23.) 



C. Horologium. — Disc orbicular, slightly saddle-shaped ; centre 

 smooth ; margin radiated. Mr. Williamson, who describes this as a 

 new species, dredged off the coast of Skye, states, the disc to be 

 less curved than the Bohemian C. clypeus, and still less so than the 

 C. zonalis of Mr. J. Phillips. Around the smooth disc is a circle of 

 short elegant projecting radii, which extend nearly to the periphery, 

 and give to the whole the general aspect of the face of a clock or 

 watch, the radii representing the figures marking the houi's. 

 Within this ring, and closely bordering tlie inner extremities 

 of the rays, is a circle of very minute and slightly elongated 

 tubercles, like those which surround the central siliceous umbo of the 

 Arachnoidiscus Japonicus, but much smaller. There are usually 

 four or five of these to each inters])acc, separating the rays. A 



