REPORT ON THE DIATOMACRjE. 129 



This inconsistency has not escaped the observation of Professor H. L. Smith, who, in his 

 synopsis of the families and genera of Diatoms, which is given in Dr van Heurck's work, 

 entitled, Le microscope, sa construction, son maniement et son application a l'anatomie 

 vegetale et aux Diatomdes, modified the definition of the genus Actinoptychus, Ehrenb., 

 as follows : — " Valves with marginal spines obsolete ; or if present, few, and in alternate 

 compartments." 



In the family of the Heliopeltece he also gives the genus Halionyx, Ehrenb., 1 to which 

 he attributes the following characteristics : — " Valves with a hyaline (stellate) 

 umbilicus, with marginal spines or teeth connected by a radial rib." But these 

 characters are precisely those of the genus Omphalopelta — a genus which Professor 

 H. L. Smith regards as synonymous with Actinoptychus, which accordingly possesses 

 spines in alternate compartments, or in each compartment — a circumstance overlooked by 

 Professor Smith. Hence it is preferable to leave the genus Actinoptychus intact, and 

 to modify Ehrenberg's definition of Omphalopelta, limiting the principal characteristic 

 to the marginal spines, whether they occur in each compartment, or in alternate compart- 

 ments. Moreover, the number of spines should be hrnited in number, in order to avoid 

 confusion between Omphalopelta and Heliopelta, Ehrenb., because in this last group the 

 numerous spines, in addition to its other characteristic marks, do not, when carefully 

 examined, present the appearance of spines, but of small spheroidal processes. 



The genus Omphalopelta may accordingly be defined as follows : — Frustulum simplex, 

 discoidale ; valvis cellulosis vel punctatis, in cuneatis dissepimentis distinctis ; area 

 umbilicali hyalina ; spinis marginalibus in omnibus vel in alternis dissepimentis." 



Omphalopelta antarctica, n. sp. (Plate VII. fig. 2.) 



Valvis areolato-punctatis, in sex partes alternatim elevatas et depressas distinctis; 

 area umbilicali parva, hyalina, indefinita ; denticulo in elevatioribus dissepimentis sub- 

 marginali ; lato margine irregulari punctulorum corona distiucto. Ad mare Antarcticum. 



This small disc, which was collected in the Antarctic Ocean, has six alternately elevated 

 and depressed compartments. The valve is delicately punctated in a quincuncial manner, 

 and also possesses distinct rounded cellules or areolae. The hyaline centre is small and 

 not distinctly defined, while a single intramarginal denticule occurs in each of the three 

 elevated segments, but is absent from the depressed segments. The margin is for the most 

 part smooth, but it is distinguished externally by an irregular circle of small points. The 

 occurrence of this circle of puncta, as well as of the rounded areolae, is especially 

 noteworthy. 



Omphalopelta japonica, n. sp. (Plate VII. fig. 10.) 



Areolato-punctata ; area umbilicali indefinita; dissepimentis pluribus (18) elevatis et 

 depressis, et denticulo marginali distinctis. In mari Japonico. 



1 Ehrenberg, Mikrogeol., pi. xxxv. A 21, fig. 12. 

 (bot. chall. exp. — paet iv. — 1886.) D 17 



