372 



Yolations increasing ratlier rapidly in dorso-veutral diameter, and more 

 gradually in convexity, compressed so as to be nearly fiat on the sides, 

 but rounding a little into the immediate umbilicus and to the periphery, 

 each so deeply embracing as to hide all the preceding ones. Surface orna- 

 mented by numerous fine, regular, nearly simple, transverse lines, which 

 increase very gradually in size from the inner to the outer whorls, and 

 in crossing the sides curve first gracefully forward near the umbilicus,. 

 then, after passing the middle, arch slightly backward, and again a little 

 forward in passing over the periphery. 



Septa extremly complex, being crowded together and very deeply 

 divided into variously-branched lobes and sinuses, which diminish regu- 

 larly in size from the peripheral margin to the umbilicus. Siphonal lobe 

 covering between one-half and one-third as much space as the first lat- 

 eral lobe, and having on each side three branches, of which the two at 

 the extremity are much larger than the others, and each divided nearly 

 to its base into two unequal slender branchlets, with several smaller 

 subdivisions and sharp digitations ; first lateral sinus a little larger than 

 the siphonal lobe, very oblique at its base, and profoundly divided at 

 its extremity into two large, unequal, slender branches, which are vari- 

 ously subdivided and sinuous ; first lateral lobe slender, but spreading 

 its deeply-divided branches over a surface nearly twice as large as that 

 occupied by the first lateral sinus, ornamented at the extremity by three 

 large, unequal, spreading branches, which are each irregularly subdi- 

 vided into from four to six or seven branchlets, with numerous pinnae . 

 and smaller digitations; second lateral sinusnearlyhalfas large as thefirst 

 lateral lobe, contracted and oblique below, and having at its extremity 

 four unequal divisions, three of which are trifid and the other bifid, and 

 all provided with numerous irregular subdivisions with sinuate margins ; 

 second lateral lobe less than half as large as the first, and divided intO' 

 about seven principal alternating branches, of which the three nearest 

 the extremity are larger than the others, and each again divided into 

 two more or less sinuous and digitate parts. 



The remaining three or four lobes diminish regularly in size, and 

 become less branched toward the umbilicus; the third, fourth, and 

 fifth being palmately divided at the extremity into five, four, and three 

 short, unequal branches, while those nearer the umbilicus are more 

 nearly simple, or only ornamented by a few small digitations.* 



In the structure of its septa, this is one of the most complex species I 

 have ever seen ; the surface of the cast being so completely covered by 

 the numerous slender branches of the lobes and sinuses as to render it 

 exceedingly difficult to follow their various ramifications. Yet, from all 

 analogy, they are doubtless even more complex in the outer whorls of 

 large adult specimens, since that before me measures only 1.90 inches in 

 its greater. diameter, and 0.53 inch in convexity; while impressions left 

 in the matrix of some of the specimens show that some individuals are 

 not less than five inches in their greater diameter. 



In form and surface-markings, it is much like Ammonites tVelledce of 

 Michelin, as figured by d'Orbigny on plate 82 of his i^alfeoutology of 

 France, vol. i (Cretaceous), though it is more compressed. Yet this 

 slight external difference, if not accompanied by well-defined peculiari- 

 ties in the lobes and sinuses of the septa, would not be sufficient to dis- 



* The typical specimen does not show the lobes and sinuses of the septa very clearly 

 beyond the siphonal and first lateral lobes and the first lateral sinus ; but Mr. Gabb'e 

 California specimen, which did not show the siphonai lobe, exhibits all of the other 

 lobes and sinuses very clearly. 



