180 CRETACEOUS TERIOD. 



axis of the sliell, nearly sixteen centimeters ; breadth of the same at right 

 angles with the former measurement, about fourteen centimeters; thickness, 

 both valves together, not far from fourteen centimeters. 



This species is remarkable for its extreme gibbosity, and the great thick- 

 ening of the test at the borders of the valves. Disconnected fragments of 

 these thicliened valves are often met with, which has led to the supposition 

 that they indicated a species, perfect examples of which were not yet dis- 

 covered, and which was provided with a test equally thick throughout. 

 The relative tliickness of the test, however, in different parts of the valve, 

 is shown at least approximately in figure 1 h, Plate XV. 



The test of all the known examples of this species, as is usuall}^ found 

 to be the case with the test of Inoceravius, has wholly a prismatic structure, 

 and is much thinner in the middle and umbonal regions than toward the 

 free borders, Avhile the reverse is usually the case with conchifers. Tliis 

 seems to indicate that a jDortion of the thickness of the A'alve at the middle 

 and umbonal region was absorbed while the animal was alive, or that a thin 

 pearly layer was originally formed upon the inside of the valves, as in 

 Pinna, and that it became dissolved or decomposed after its entombment, by 

 agencies which the prismatic laj^er resisted. The latter suggestion is strength- 

 ened by the fact that many of the Cretaceous Pinnas show no trace of a 

 pearly layer, such as living Pinnas have, while the prismatic layer is well 

 preserved. It is difficult, however, to reconcile this with the fact that the 

 pearly layer in I. Barahini is often as well or better preserved than the pris- 

 matic layer. 



Position and locality. — Strata of the Cretaceous period ; five miles above 



Pueblo, Coloraclo. 



Inoce ramus Barabini Morton. ^ 



Plate XVI, fig. 4 a. 



Inoceramus Barahini Morton, 1834, Synop. Org. Eemaius Cret. Group, 02. 



? Inoceramus Crispii Courad, 1857, U. S. & Mex. Bound. Surv., i, 152 (not Muntell). 



Shell of medium size, elongate-ovate in marginal outline, moderately 

 inflated, especially in front and in the umbonal regions ; A-alves equal, or 

 nearly so ; umbones more or less inflated ; beaks small, not reaching quite 

 so far forward as the front side of the shell ; front truncated and short ; 

 basal margin broadly rounded; posterior margin curving somewhat abruptly 



