342 THE LIAS AMMONITES. 



known that it was to be found in older beds in Robin Hood's Bay. Mr. Simpson^ says " I 

 have now got a fair specimen of Sowerby's A. armatus which has caused so much trouble 

 to naturalists, the very central whorls are imperfect, but there is sufficient to show that they 

 are plain or nearly so ; the succeeding whorls have the characters exhibited in Sowerby's 

 figure. Where the spines have been knocked off there remain the oval disks. It would seem 

 that when Mr. Strangewayes visited Whitby this species was plentiful ; but the specimen I 

 have now described is the only one I have seen, and the one I formerly took to be A. 

 armatus, Sow., I believe to be a different species, which I have named A. miles!' 



Like other species of the Armati section of the genus Aegoceras, Aeg. armafum exhibits 

 different forms characteristic of the morphological phases of its growth. In early life the 

 shell is smooth, and about the second whorl ribs commence to show themselves as transverse 

 elevations, and on the fourth whorl small tubercles begin to make their appearance on their 

 sides, and on the sixth whorl they have grown into large, thick, stout spines (PI. XXVIII, 

 fig. 3). The magnificent specimens figured in PI. XXVIII, figs. 1, 2, 3, 4, and 5, represent 

 the form this beautiful Ammonite assumes in middle age, when its ornamentation appears to 

 have attained its most perfect state. Now we find the whorls are very slightly involute, a little 

 rounded, and provided with thick ribs, which terminate on the outer side of the siphon al 

 area in twenty strong prominent spines ; these, when broken near their base, leave large 

 disc-like marks on the mould ; between the ribs are several (four or five) transverse striae 

 which pass between the ribs, extend across the area (figs. 2, 4, 5), and ornament this 

 region ; other smaller striae creep over the spines themselves (fig. 3), so that the whorls 

 in well-preserved specimens are finely sculptured with graceful lines. Through increasing 

 age the spines are developed wider apart, the body-chamber grows very wide, and 

 enormous recurved spines project from the sides of the siphonal area. I have figured a 

 very fine example of one of these aged specimens of the natural size (PI. XXIX), showing 

 giant spines on the body-chamber and the comparative smallness of those on the inner 

 whorls. This specimen was collected at Lyme Regis many years ago, and long adorned 

 the cabinet of a local collector. It was purchased for the Museum of the Royal School 

 of Mines and now forms part of their fine collection ; it is figured for the first time. 



Professor Quenstedt collected this Ammonite in Swabia, where the species is much 

 smaller than our English specimens, but is well defined by the dehcate folded striae or fine 

 concentric ribs which extend along the sides and across the area, among which, near the 

 margin, large thick spines project on both sides, fewer, however, in number than in our 

 specimens. 



There are several varieties of this species which have received separate specific names 

 from different authors. One of these, A. miles, Simpson, deserves more than a passing 

 notice ; the spire in this shell consists of six highly evolute whorls, which are round, 

 slender, and all exposed in the umbilicus ; the ribs, about sixteen in the last whorl, are wider 

 apart, and each rib terminates near the siphonal margin in a long pointed spine ; the lateral 



1 ' Fossils of the Yorkshire Lias,' p. 64, 1855. 



