Cretaceous and Tevtianj Cirri/^edes. 173 



Description of voices. All the valves of this species arc 

 conspicuously marked on their outer surface with sharp, 

 narrow, stccp-sidcd, prominent ridges parallel to the growth- 

 lines, and each of these ridges appe.irs to have been formed 

 at the completion of each pcrioii of growth. In some of the 

 specimens from the Alblan (Gault) the spaces l)etvveen the 

 ridges are smooth, and iu others they are plainly marked 

 Mith longitudinal ridges ; hut, since all the specimens seen 

 from the Cenomanian (Chalk jNIarl) have longitudinal ridges 

 and the ornament altogether is more pronounced, it seems 

 as if the valves from the Gault with smooth interspaces came 

 from a lower horizon than those with lidged interspaces. 

 All the valves from the Clialk Marl are very much smaller 

 than the valves from the Gault. The following descriptions 

 are based on valves from the Gault of Folkestone, but any 

 distinctive features shown by the valves from the Chalk 

 ;Marl are pointed out where considered necessary. 



Carina (PI. VII. fig. 19) semieylindrical, widening gradu- 

 ally from the apex to the basal margin, moderately bowed 

 inwards, strongly convex transversely, not earinate, basal 

 margin slightly concave iu the middle. Outer surface orna- 

 mented with a number of irregularly spaced, raised, and 

 somewhat undulating ridges, which on the extremely narrow 

 parietes are obliquely upturned ; in some specimens the 

 spaces between these ridges are smooth, but in others, espe- 

 cially in those from the Chalk Marl, they are plainly marked 

 with longitudinal ridges. The apical half of the valve pro- 

 jected freely, and on the inner surface this part of the valve 

 is marked with growth-lines which extend from the basal 

 angles and meet iu an acutely rounded angle on a slight but 

 well-marked median ridge. 



Rostrum (PI. VII. fig. 15) semiconical, smaller and pro- 

 portionally wider than the carina, widening rapidly from the 

 apex to the basal margin, considerably bowed inwards, 

 strongly convex transversely, basal margin concave. Outer 

 surface ornamented similarly to the carina. The apical half 

 projected freely, and on the inner surface this part is marked 

 with growth-lines which extend from the basal angles and 

 meet below the apex in a wide flatlv rounded angle. 



Scutum (PI. VII. fig;. IG ; Pl.'VIII. fig. 4) elongately 

 triangular, with the basi-lateral portion produced, strongly 

 convex transversely, especially in its apical portion, apex 

 acuminate and strongly curved towards the terga ; occludent 

 margin strongly convex ; basal margin about half the length 

 of the occludent margin, and making Avitli it an angle con- 



