OF THE COSCINODISCEiE. 



841 



young state. Ehrenberg has dedicated the different species of this genus to 

 persons distinguished in the history of microscopic research " (Bailey). As 

 the species differ only in the number of compartments, they are probably not 

 truly distinct. 



Heliopelta J/f^n (E.). — Disc having 

 loosely cellulose, elevated, radiating com- 

 partments, alternating with depressed 

 ones marked with tine decus^sating lines; 

 border a rather broad striated rim. Ber- 

 muda deposit, (xi. 35.) Compartments 

 6; umbilicus stellate. Diam. 1-372". 

 Has the habit of Actinoptijchus velatus. 

 = H. Metii, EB. 1844, p. 268. Com- 

 partments 8 ; umbilical star tetragonal. 

 Diam. 1-204". 



H. Leeiiwenhoekii (EM. pi. 33. 18. f. 5.). 

 — Compartments 10 ; umbilical star pen- 



tagonal. Diam. 1-156". 



H. Eideri (EM. pi. 33.^ 18. f. 6.). — 

 Compartments 12 ; umbilical star hex- 

 agonal. Diam. 1-156". 



H. SeUiquerii (EB. 1844, p. 268.). — 

 There are usually 3 teeth opposite each 

 elevated compartment, and 2 opposite 

 each depressed one ; but sometimes, es- 

 pecially in the larger specimens, the teeth 

 are more numerous, whilst in the smaller 

 ones they are occasionally 1 less in each 

 compartment. 



Genus OMPHALOPELTA (Ehr.).— Fmstules simple, disciform ; disc cellu- 

 lose or punctate, divided by imperfect septa into cuneate rays ; centre hyaliue ; 

 spines, one to each compartment. " This genus has the habit of Actinoptychus 

 and Heliopelta, but differs from the former in the presence of lateral spines, 

 and from the latter in the small number of these processes. The species of 

 these three genera often closely agree in their form as well as in the number 

 of their radii and cells ; but the character of the spines will always distinguish 

 them" (Bailey). '' All the species of Omphalopelta resemble Actyno^tyclms 

 senarius^' (Kg.). 



Heliopelta differs from this genus in having two or more spines instead of 

 one to each compartment, — a difference we regard" as more suitable for specific 

 than generic distinction; and we believe that a better knowledge of these 

 forms will prove the propriety of uniting them. 



Omphalopelta cellulosa (E.). — Ra- 

 diating compartments 6, cellulose, al- 

 ternately tumid and depresed, stellato- 

 punctate ; rays but slightly prominent ; 

 rim broad, striated. KA. p. 133. Fossil. 

 Bermuda, Virginia. Diam. 1-192". This 

 species greatly resembles the 6-rayed 

 form of Heliopelta 3Ietii, in which the 

 compartments have sometimes only 1 



and 2 spines alternately ; and indeed we 

 are not certain that they are even speci- 

 fically distinct. 



0. areolata (E.). — Compartments 6, 

 all loosely and obscurely cellulose, 

 scarcely or but slightly depressed; rays 

 distinct ; rioi broad, radiate. EM. pi. 35 a. 



18. f. 12. = Actinocydus areolatus, Bri MJ. 

 viii. p. 93, pi. 5. f. 1. Fossil. Bermuda, 

 guano. {\Tii. 15.) 



0. ve)'sicolor (E.). — Compartments 6, 

 all granulated in very fine decussating 

 lines, which cause a play of colours from 

 tawny to red ; the strong rays and hex- 

 agonal crystalline umbilicus ^ery conspi- 

 cuous ; rim naiTOw, radiant. KA. p. 133. 

 Fossil. Bermuda. Diam. sometimes 

 1-252", but mostly less. 



0. pu7ictata (E.). — Radiating compart- 

 ments 6, all loosely punctated, 3 alter- 

 nate ones slightly elevated ; rim narrow, 

 not distinctly radiant; spines obsolete. 

 KA. p. 133. Fossil. Bermuda. 



Genus ARACHNOIDISCUS (Deane).— Frustules disciform ; disc with a 

 central hyahne nodule or umbilicus, and numerous radiating liaes connected 

 by concentric lines or series of gemmaceous granules. =Hemiptychus (E.). 

 The disc has been compared to a spider's web ; hence the name. Alternating 

 with the long radiate Hues are one to thi^ee short marginal ones, the central 

 one of these being also longer than the other two when three are present. 

 Professor Bailey informs us that Arachnoidiscus has been adopted instead of 

 Hemiptychus because the latter name had previously been used in ento- 

 mology. 



