244 



to 30, obtuse, vanishing near the carinal sulci ; lateral septae 3 lobed ; 

 shell covered on the back and sides with fine longitudinal lines, 28 on 

 each side of the whorl, producing a series of p\inctuations where they in- 

 tersect the transverse lines of growth. 



Dimensions. — The usual size is from 6 to 8 inches in diameter, but it 

 attains 12 to 14 inches. The relative diinensions calculated from several 

 specimens are — height of the last whorl, -^-^; width, -j-Voi "width of the 

 umbilicus, -^Vo '> involution of the whorl, -rVo of the diameter of the shell. 



Description. — This Ammonite has a discoidal shell, moderately com- 

 pressed on the sides, but more so towards the back, which is furnished 

 with a very thick obtuse keel, having on each side a deep sulcus separ- 

 ating it from the two lateral carinae formed by the angles of the sides and 

 back ; mouth compressed, bisinuated at the summit, and strongly grooved 

 by the turn of the spire ; whorls slightly compressed in the middle, and 

 more so towards the upper third ; provided with from 25 to 30 moderately 

 prominent acute ribs, which are arched, and disappear at the angle 

 of the back before they merge into the lateral carinal sulci ; the valleys 

 between the ribs are very regularly concave. 



The entire surface of the shell is traversed by fine longitudinal lines, 

 as if they had been cut by a graver ; on the specimen before me there 

 are sixty-six such lines on the back and sides of the whorl, and at the 

 point where they intersect the transverse lines of growth a punctuated 

 depression is produced, which imparts an ornamental character to the 

 shell of this species. 



The septse are symmetrical, divided on each side into four lobes and four 

 saddles, formed of unequal parts. The dorsal lobe is long and wide, and 

 divided on the median line one-third of its length, presenting numerous 

 small equal-sized digitations around its circumference ; the dorsal saddle 

 as large as the dorsal lobe, has shallow ramifications on its outer side, and 

 three small festoons on its anterior border ; the superior lateral lobe is 

 conical, and about two-thirds the length of the dorsal lobe ; the superior 

 saddle is larger than the dorsal saddle, and presents some better developed 

 simple festoons on its border than those on the latter ; the inferior 

 lateral lobe is about as large as the superior, and presents numerous 

 small rounded digitations on its circumference ; the auxiliary saddle 

 is about one-fourth the size of the dorsal, and has a similar form ; the 

 auxiliary lobe is nearly as large as the infero-lateral. 



Affinities and Differences. — This species very much resembles Ammon- 

 ites stellaris, Sow. ; so much so, that the one is often mistaken for the 

 other. Both have longitudinal lines on the shell, and punctuations at 



