LOWER CRETACEOUS FOSSILS — ETHERIDGE. 147 



The costae of the specimen mentioned in the latter portion of 

 the last pai-agraph but one, are one inch apart, and as the shell 

 increases in age the sigmoid al curvature becomes less apparent, 

 and the costae straighter or less curved. On these mature whorls 

 these large costae are more abruptly inclined on their fore aspects, 

 gradually sloping or shelving off on their hinder faces into the 

 concave valleys or intercostal spaces ; this is a characteristic 

 feature of C. jackii. 



The costae of the tuberculate whorls, and for some distance 

 beyond the termination of the nodes, are uniformly convex, 

 obtuse and close. Those bearing tubercles are always more or 

 less swollen and enlarged, with from two to four simple ribs 

 between them. The normal number of rows of tubercles is 

 certainly three, as already described, but specimens are before 

 me with two, or even only one row visible. At the same time it 

 may be accepted as a fact that in no instance has a specimen 

 with the general characters ascribed to this species been found 

 to possess nodes beyond about the third or certainly the fourth 

 whorl. 



The section of the tubercle whorls is octagonal, the venter 

 between the two abdominal rows being truncate and hollowed. 

 The section of the remainder of the whorls is transversely deltoid. 



The convexity of the flanks and the marked difference in 

 sectional outline at once distinguishes C. jackii from the portions 

 I have termed C. leptus. 



To some extent in its young state it is our representative of 

 the magnificent European C. romeri, N. & XJ.," 1 but the latter is 

 much more highly ornate and the cost* of the two forms differ 

 otherwise. In general appearance C. jackii is not unlike the 

 American form C. percostaticm, Gabb 32 ; a comparison may also 

 be made with C. ramososeptatum, Anthula, 33 in which the first 

 three whorls are also nodose. 



In a previous paragraph it is stated only one row of tubercles 

 may occur on the earliest volutions of this species. Indeed, one 

 may go a step further by calling attention to specimens in 

 which no trace of tubercles is visible, but otherwise presenting 

 the costal features of the more mature conditions of C. jackii. 

 These costae are remarkably regular, thick, rounded, although 

 rather depressed, particularly on the venter, where they are 



31 Neumayrand Uhlig — Palaeontographica, (3), xxiii.,188l,3,p. 187, pi. lv. 



a2 Gabb— Pal. California, i., 4, 1861, pi. xvi. 



3:5 Anthula — Beitriige Pal. Oster.-Ung. Orients, xii., 1900, pi. xiv., f. 4a. 



