404 THE LIAS AMMONITES. 



shell, which has been mistaken for a third tubercle ; the sides have wide concave valleys 

 which separate the ribs from each other and clearly define their structure and character. 



The whorls are slightly involute, and the umbilicus is therefore widely open, exposing 

 the straight sides of all the ribs with their bituberculated eminences on each, and hence 

 the umbilicus of this Ammonite has a highly ornate interior, which, together with the 

 breadth of the siphonal area, and its bold, oblique ribbings and carinated keel, cause this 

 species to be one of the handsomest shells in the Lias formation, as may be seen in this 

 figure on PI. LV. The aperture is nearly quadrangular, it presents two sinuses on its 

 external border, which are separated by the median carina, and the inner border is slightly 

 indented by the return of the spire (PI. LV, figs. 1 and 2, and PI. LVI, figs 3 and 5). 



The sutural-line is rather complicated, and forms three lobes and as many saddles. The 

 siphonal lobe is as long but not so wide as the principal lateral (PI. LVI, fig. 4), it is 

 ornamented on each side with four lateral branches and a terminal tuft of digitations. 

 The siphonal saddle, wider than the principal lateral, is rounded above and divided into 

 three accessory lobules, their size augmenting in length and width from without inwards. 

 The principal lateral lobe is wide below, and ornamented with five branches, two lateral 

 and one large terminal, each having several simple digitations. The lateral saddle is much 

 smaller than the principal lateral lobe, and terminates in one round terminal and two 

 lateral, unequal folioles on each side ; of the two accessory lobes the external is the 

 longest, and consists of a stem with several simple digitations, the internal is short and 

 terminates in a point. The auxiliary saddles are very small, and terminate in two folioles. 



The morphology of this species, when traced by a series of specimens, is very interest- 

 ing. Up to a size of 2 — 3 millimetres the shell is entirely smooth with a round 

 siphonal area, and its sides are covered with short, straight ribs ; the siphonal area 

 remains round until it attains 7 millimetres in diameter. The median line now becomes 

 slightly prominent, and when a diameter of 1 2 millimetres is attained a keel makes its 

 appearance. When the shell has grown to 34 millimetres in diameter, it has acquired 

 all its ornamentation, and at 54 millimetres the row of external tubercles is very large 

 and prominent. At about 180 millimetres (PI. LV) it has attained its full measure 

 of development ; as old age draws on the ribs are not so numerous, and the tubercles are 

 less prominent. There are some differences in the form of the shells which may probably 

 depend upon sexual characters, such as a greater or lesser flattening of the shell in 

 certain individuals, but as this is a problematical subject, I merely touch upon it here, 

 as we have no facts on which to found any decided opinion. 



Affinities and Differences. — In early life this species very much resembles Amaliheus 

 margaritatus, but as growth proceeds its specific characters gradually evolve themselves, 

 and finally new lines of development separate each form into its own special type, the narrow 

 siphonal area of AmaL margaritatus with its deeply carinated keel, the longitudinal lines 

 of punctations and lateral undulatory folds on the sides, produce a form widely different 

 from the typical Amaltheus spinatus figm-ed in PI. LV. 



