74 On Ancient Water-Jieas of the 



portant feature in the rhomboidal outline of the valves {Rlionibina), 

 which are rare in the same rock. The " Monitor " group is named 

 Cypridinella, with seven species ; its most symmetrical and ovate 

 form is C. Cummingii ; some showing the extreme of its prow- like 

 feature are C. monitor and C. vomer. All occur in the Carbonife- 

 rous Limestone of Europe and the British Isles. 



In the next group, Cypridellina, we have the form of Cypri- 

 dinella (for which, hy-the-by, we have no near recent represen- 

 tative) with a superadded feature, namely, a subcentral tubercle, or 

 swelling at or near the centre of each valve. There are eight 

 species, with several varieties all like the foregoing, from the 

 Mountain Limestone. Some few of these closely imitate Cyp'idi- 

 neUa, others go off in divergence of shape, especially in the prow, 

 which inclines to be vertical. 



When, in addition to the tubercle, a nuchal furrow is present, 

 we see the CyprideUa of De Koninck (revised) : for among the 

 associated fossil forms there are several very closely related to 

 Cypridina in general characters, but differing from it in having 

 the faces of the valves raised up in one or more tubercles, and in 

 being impressed near the middle of the dorsal region by a short, 

 vertical, and often curved sulcus, generally immediately behind the 

 chief and most persistent tubercle.* The tubercles may be three 

 or four in number, giving the valves an irregularly quadrate shape. 

 Usually there is only one tubercle, at or about the centre of each 

 valve, and even that may be almost obsolete; and so also the 

 furrow is sometimes so faint as scarcely to be recognized. The 

 notch and peak are usually large and distinct. These forms, which 

 are exceedingly variable, lie under Cypridella, a name given by 

 De Koninck to one of the most marked of them (C. cruciata, not 

 yet found out of Belgium). C. Edvardsiana {Cypridina, De 

 Koninck) resembles a Cypridinella in shape, but is swollen here 

 and there into tubercles, fewer in the young than in the old 

 state, and is impressed also with the nuchal sulcus. We have 

 figured G. Koninckiana,^ in which, as usual, there is but one 

 tubercle. In De Koninck's Cypridella cruciata the tubercles and 

 dorsal sulcus are very strong, and a subquadrate outline results. 

 There are gradations through C. Wriglitii and C. obsoleta, to the 

 smooth ovate forms of Cypridinella, and even to the acute ovate 

 outline (in C. cyprelloides) found in the next group. Cypridella 

 belongs to the Mountain Limestone of Europe and the British Isles. 

 Sulcuna, from the same limestone, presents some few forms cha- 

 racterized by a general resemblance to some Cypridellse, but so 

 deeply indented by an oblique dorsal sulcus as to present sloping 

 outstanding processes on the antero-dorsal regions. 



* As in rrimitia, see before, vol. iv., ji. 191. t Tliilc LXI., Fig. 9, vol. iv. 



