OCR Text |
Show 372 volutions increasing rather rapidly in dorso- ventral diameter, and more gradually in convexity, compressed so as to be nearly flat 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 ornamented 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 BOM* 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 regularly in sine from the peripheral margin to the umbilions. Siphonal lobe covering between one- half and one- third as nnuob space as the first lateral lobe, and having on each side throe branches, of which the two at the extremity are much larger than the others, and each divided aearly to its base into two undqual slender branohlets, with several smaller subdivisions and sharp dictations; first lateral sinus a little larger than the eiphonal lobe, very oblique at its base, and profoundly divided . at its extremity into two large, unequal, Blender branches, which are variously 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 subdivided into from four to six or seven branchiate, with numerous pinme andsmaLlerdigitsAions: second lateral sinusneariy halfae large as thefiret lateral lobe, contracted and oblique below, and having at its extremity four unequal divisions, three of which are trifid and theotiher 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 pajmately 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 oomplex species I have evBr 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 individuate are not less than five inches in their greater diameter. In form and surface- markings, it is much like Ammonites Velleda of Michelin, as figured by d'Orbigny on plate 82 of his palaeontology of France, vol. i ( Cretaceous), though it is more compressed. Yet this slight external difference, if not accompanied by well- defined peculiarities in the lobes and sinuses of the septa, would not be sufficient to dis- 4 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 sinns; but Mr. Gablrt California specimen, which did not show the eipbonaf lobe, exhibits all of the other lobes and sinnses very clearly. |