AEGOCERAS DENSINODUM. 351 



Description. — Ammonites armatus, according to Professor Quenstedt, presents two 

 well-marked varieties : a, Armatus sparsinodus, and b, Armatus densinodus. After a 

 careful study of the latter shell, which I have collected in different localities and in con- 

 siderable numbers, I have erected it into a distinct species, as it exhibits characters which 

 I have found to be specific and permanent. 



The shell is discoidal, compressed and slightly involute, with a wide umbilicus, all 

 the whorls being fully exposed in the medium-sized specimen (PL XXXIX, figs. 6 and 7). 

 The flat sides are ornamented with twenty-five to thirty oblique ribs, which become visible 

 above the spiral suture. Near the middle of their length they develop a small tubercle 

 (fig. 8) ; beyond this the rib enlarges, and terminates at the margin of siphonal area in a 

 round, blunt, prominent tubercle. In most of the specimens which have passed through 

 my hands the ribs are inclined obliquely backwards, on which the double row of tubercles 

 are very well developed (fig, 8). In other specimens, however, the marginal tubercles 

 alone are developed, and the position of the inner series on the middle of the rib is 

 indicated by a simple thickening of the lateral fold. The siphonal area is narrow, 

 flat (figs. 7 — 9), and crossed by numerous transverse striae ; some of these pass from 

 one large tubercle on one side of the area to those of the opposite side ; in the inter- 

 tubercular space three small striae pass across, so that the area has a highly ornate 

 appearance in well-preserved specimens, as in figs. 8 and 9. 



In the large specimen, PI. XXXVIII, figs. 5 and 6, and in a smaller individual of the 

 same type, PI. XXXIX, fig. 6, and PI. L, figs. 11, 12, the medium row of tubercles is 

 absent, and the ribs have the oblique backward inclined curve so characteristic of this 

 species in the large specimen, which is exceptionally fine ; and the termination of the body- 

 chamber is greatly enlarged and forms a trumpet-mouthed expansion, which is likewise 

 characteristic (PI. XXXVIII, fig. 5) ; the body-chamber has a quadrate shape, the area is 

 flat, and the whorl ornamented with thirty-eight ribs, the whole structure of the spire 

 being exposed in the widely open umbilicus. The lobe-fine is very complicated (PI. 

 XXXIX, fig. 10). The siphonal lobe is large and symmetrical, with three digitations 

 on each side and two long terminal branches having serrated sides. The siphonal saddle 

 is wide, and divided into three compartments, the internal and external being deep and 

 terminating in folioles, and the central one narrow from the size of the partition. The 

 principal lateral lobe is long, narrow, and branched ; it has an irregular figure, the terminal 

 portion ending in a long point. The lateral saddle is deep, terminating in two folioles like 

 two oak leaves, with a narrow stem between them. The lateral lobe is long, narrow, and 

 oblique, with a freely digitated margin. The auxiliary saddle is small and oblique, and ends 

 in one foliole, and the auxihary lobe is likewise small and oblique with lateral digitations. 



The morphology of this species is very interesting. The facts I have noted were 

 obtained from the study of a large assemblage of the brood of this species, collected 

 from a thin bed in which they were entombed. In the first three volutions the young 

 shell is quite smooth ; in the fourth sharp ribs make their appearance with concave spaces 



