94 Dr. T. Wright oji a new Genus of Fossil Cidaridse, 



and surface-elevations, or even their absence or presence, is not 

 sufficient to distinguish species, as shown by Thwaites*, &c. and 

 Shadboltt. The observations of llalfsj upon Himantidium are 

 very interesting in this respect, although unfortunately very 

 incomplete. 



It may be remarked, that the surface of the valves of the 

 Navicula described above were covered with minute depressions, 

 invisible by ordinary illumination, but visible by oblique light 

 with the use of stops ; while the sporangial sheath was free from 

 markings, save the transverse striae. 



The manner in which the unyielding siliceous sporangial 

 sheath enlarges in accordance with the growth of the sporangial 

 frustule is obscure, unless it ensue by the occasional ecdysis, as 

 it might be called, of the old sheath and the formation of a new. 

 In the earliest conjugating specimen observed of the above Navi- 

 cula, the sporangial sheath was of a rounded-oblong form, about 

 two-thirds of the size of the parent frustules, and situated midway 

 between them, both in regard to length and breadth, whilst 

 the full-grown sporangial frustule and sheath were twice the 

 length of the parent frustule (figs. 2 & 3), both the former being 

 of about the same size, and the latter situated near one end of 

 the sporangial sheath. It was noticed also, that all the spo- 

 rangial frustules were undergoing division, although contained 

 within their sheaths. 



VIII. — On a New Genus of Fossil Cidaridse, with a Synopsis of 

 the Species included therein. By Thomas Wright, M.D., 

 F.R.S.E. 



Notwithstanding the many new generic sections introduced 

 into the classification of Echinoderms, by MM. Agassiz and 

 Desor, and the important light thrown by these savans on our 

 knowledge of the numerous species of this class contained in 

 European collections, still the progress of discovery renders it 

 imperative on palaeontologists to modify from time to time 

 many of the opinions put forward by these authors in their 

 '* Catalogue raisonne.' When the amount of real work done by 

 them is taken into account, in a field which was then compara- 

 tively unknown, the wonder is, not that mistakes or oversights 

 should have been committed, but that so much good work 



* Annals, loc. cit. 



t Shadbolt, Microsc. Journal, vol. iii. 



X Ralfs, Annals, 1843, xii. on Fragilaria pectinalis, U. {Himantidium 

 pectinate, Kiitz.) var. j3. 



