REPOKT ON TEE DIATOMACI'LE. 133 



been accepted, as it is not sufficiently distinct from that of Asteromphahis, to which the 

 forms in question accordingly remained annexed. 



Asteromphahis ovatus presents, however, several points of resemblance to the Asterom- 

 phalus shadboUianus of Greville, 1 who described this latter Diatom as an Asterolampra, 

 having endeavoured to join the two genera Asteromphahis and Asterolampra into one 

 under the latter name, because by supposing one radius of Asterolampra to remain unde- 

 veloped there would be left no means of distinguishing them. This supposition, however, 

 has failed to meet with the acceptance of other micrographers. The points of difference 

 which may be noted between the frustule now before us and Asteromjihalus shadboUianus, 

 Grev., are the following: (1.) The form of the umbilical lines which proceed from the 

 superior margin of the pyriform area of the obliterated radius are angularly curved and 

 almost broken in the Grevillian species, while in the present form they are straight, with 

 the exception of those separating the inferior radial areas, which are slightly curved, but by 

 no means bent in a zigzag manner, as in the other case. It is to be remarked, however, 

 that the occurrence of a bending in the umbilical lines is of little significance, because all 

 gradations may be found in the same species, from the most decided curves to simple 

 lines which are but slightly tortuous or may even be straight. (2.) The present form is 

 smaller than the species established by Greville; and (3.) the former, instead of being 

 round like the latter, is oval. From these considerations the specific value of Asteromphahis 

 ovatus cannot be questioned. 



Asteromphahis roperianus, Grev., var. atlantica, nov. (Plate V. fig. 3.) 



This frustule also belongs to the group designated Spatangidium by Brebisson. It 

 presents a beautiful granulated disc on which a hyaline star with six radii — 'apart from 

 that which is obsolete — occurs. It is very closely allied to Asteromphahis roperianus, 

 Grev. : " in both there is the same number of radii, of interradial areas with straight 

 vertices, and the central area of the obsolete radius, at which the other umbilical fines 

 meet, is campanulate in both. It differs from Greville's species, however, in the following 

 respects : (1.) The extremities of its radial areas are slightly expanded, and terminate 

 some distance from the margin of the frustule. (2.) The granulated segments at the 

 apices terminate abruptly and not in a somewhat undulating manner, as in the figure 

 o-iven by Greville, who represents two slightly convex curves as starting from the two 

 angles of the apex and meeting in a slight involution at the intermediate umbilical line. 

 Notwithstanding these points of difference, however, the present frustule can only be 

 regarded as a variety of Greville's species, and it has been named atlantica from the locality 

 in which it was found. 



1 A monograph on the genus Asterolampra, including Asteromphalus and Spatangidium, by R. K. Greville 

 in Micr. Jonrn., vol. viii. p. 121, pi. iv. fig. 19, 1860. 



2 Micr. Journ., vol. viii. pi. iv. fig. 14, 1860. 



