WHITEAVES.] CRETACEOUS FOSSILS, NORTH WEST TERRITORY. ITI 



SCHLOENBACHIA GRACILIS. (N. Sp.) 

 Plate 23, figs. 2 and 2a. 



Shell nballowly but widely umbilieiitccl, tbe inner whorls occupying :i 

 little more thun one half of the entii'e diameter: periphery encircled 

 apparently with a central and very slightly raised, simple abdominal 

 keel. Volutions about tive, narrow, increasing slowly in size and but 

 slightly embracing. Outer volution somewhat rectangular, its sides 

 being compressed and its abdominal region slightly flattened on both 

 sides next to the keel, though r..iunded otF exteriorly. Aperture subel- 

 liptical but somewhat rectangular, higher than wide and very slightly 

 eniarginate by the encroachment of the preceding volution. 



Surface marked by distant, slightly curved, prominent and simple 

 radiating ribs or rib-like folds. On the outer volution these ribs, which 

 curve somewhat obliquely forward in a shallowly concave curve, become 

 obsolete in the abdominal region and disappear before reaching the 

 keel. They are much narrower than the broad, shallow depressions 

 between them, and are most pi'ominent a little moi'c than half way 

 across the sides, where each rib rises gradualij^ into a low, pointed 

 tubercle. Sutural line unknown. 



Appn-oximate dimensions of the most perfect specimen collected • 

 greatest diameter, one hundred and ten millimetres ; width of umbilicus, 

 from suture to suture, fifty-eight mm.; height of the aperture (at the 

 broken anterioi- extremity, thirty mm.; width of the same, if measured 

 on the summits of two opiposite ribs, twenty-two mni., or, if in the 

 interstices between them, eighteen mm. 



Two imperfect and not very well jjreserved specimens, both of which 

 are mere casts of the interior of the shell. 



Associated with these, two other fragments of a Schloenbaclda were 

 collected by Mr. McConnell, in one of which the abdominal region is 

 encircled by three prominent angular ridges, of which the central one 

 is much the highest, just as in the Ammonites Tehamaensis of Gabb,* 

 which would now be called a iSchloenbachia. In several other respects, 

 however, these fragments differ materially from the tj'jies of S. gracilis 

 and from S. Tehamaensis. Their volutions are much more closely em- 

 bracing, their outer volution is much broader in a dorso-ventral direc- 

 tion, and their umbilicus is much narrower in proportion to the size of 

 the shell. The ribs on their outer volution, too, are very faint and in- 

 distinct, and in none can any trace of a tubei'cle be detached either 

 near their centre or at their outer termination. 



* Geol. Surv. Calif., PalEeont. Vol. II [1869) p. 132. 



