216 Prof. T. R. Jones and Dr. H. B. Holl on 



other, showing that the outside of the shell is smooth, and its 

 interior moulded with the undulated contours that we see on the 

 very common casts above mentioned. The undulations of the 

 surfaces of the casts are caused by two oblique transverse sulci, 

 of variable depth and width, rather nearer to the anterior 

 (narrow) than to the posterior or boldly convex extremity, 

 and deepest and broadest on the convex (dorsal) edge. The 

 space between the two furrows is marked out more or less 

 plainly as an obliquely oblong, suboval, or roundish lobe ; 

 but whether faint or strong, it has no corresponding elevation 

 on the outside of the shell ; nor have the furrows on the cast 

 anything to indicate them externally. 



The smooth, bean-like exterior (now recognized) entirely 

 distinguishes the form under notice from Beyrichia ; and even 

 the fuii'ows in the cast ought to be largest near the straight, 

 and not at the convex margin, for a true Beyrichian character. 



Of existing genera, Cytlierella is the only one that has 

 its carapace-valves smooth externally and excavated on 

 their inner face. The form under notice -has a thick shell, 

 as is usual with Cytlierella ; but the internal hollowings in 

 the shell, giving rise to the three obscure lobes of the cast, 

 differ much in shape from those of Cytlierella ; whilst the 

 valves are less oblong and more convex than those of this 

 last-mentioned genus ; the overlap of the left valve and the 

 incurved ventral edge are also distinctive. We therefore pro- 

 pose a new genus for its reception, with the name Cytherellina, 

 and with the following definition, limited by want of mate- 

 rial : — 



Carapace-valves elongate, convex, smooth, thick, excavated 

 internally with undulating contours. 



CytJierellina siliqua, Jones, sp. PL XIV. figs. 1-6. 



Beyrichia siliqua, Jones, Aun. Nat. Hist. ser. 2. vol. xvi. p. 90, pi. 5. fig. 22 

 (1855). 



A subcylindrical, smooth, bean-shaped Cytheroid cara- 

 pace, boldly convex behind, somewhat tapering in front, in- 

 curved at the anterior third of the ventral border ; strongly 

 and gracefully arched on the back, with a rapid slo])e to the 

 front. The dorsal arching varies to some extent in different 

 individuals. The left valve overlaps the other on the ventral 

 margin, and in some degree also on the anterior and posterior 

 borders. 



Carapace- valves elongate-ovate, contracted at one end (an- 

 terior), boldly rounded at the other ; arched on the back, nearly 

 straiglit below; internally thickened by two oblique, trans- 

 verse, low ridges near the centre (or, conversely, impressed 



