AEGOCERAS HENLEYI. 367 



Reinecke, for Aeg. Henleyi, Sowerby, although the two fossils are sufficiently distinct 

 from each other, Aeg. striatum having large, highly involute whorls, a narrow umbilicus, 

 with a constant uniform growth and ornamentation, whilst Aeg. Henleyi has a slightly 

 involute shell with a wide umbilicus, and changes its outward whorl in a remarkably 

 sudden manner at about its sixth volution. In this stage of its growth it much resembles 

 Aeg. striatum ; and, as the inner whorls are seldom preserved with the outer whorl in con- 

 sequence of the evolute character of the shell, the two species have been usually grouped 

 together and known in most works as Aeg. Henleyi, whilst Reinecke's capital species has 

 been entirely lost sight of. 



In early age this Ammonite has a compressed discoidal shell, with slightly involute 

 whorls, flattened on the margin, and ornamented with simple, obtuse ribs on the sides, 

 each having two small tubercles ; the inner row very small, the outer row larger, and 

 both can be detected with the finger when they are seen with difficulty. From the outer 

 tubercles the rib forms a prominent arch over the siphonal area with deep valleys between. 

 In this condition it is figured and described as Aeg. latcecosta. In the adult state the shell 

 is entirely transformed by the sudden enlargement of the body-chamber (fig. 2), the 

 ribs of which become much smaller and more numerous, and the two rows of tubercles are 

 enlarged and form conspicuous objects in the ornamentation of the shell ; those of the inner 

 row remain small, and those of the outer row grow more elongate ; and from each the rib 

 splits into two or more branches before it crosses the wide siphonal area, which region is 

 thus highly sculptured by the numerous fine lines that traverse it from side to side 

 (fig. 3). A change of form in the morphology of the shell is observed in most Ammo- 

 nites at diS"erent periods of their lives, as we have seen in Aeg. planicosta, Aeg. biferum, 

 and others ; but in none is the transformation scene so rapid and complete as in Aeg. 

 Henleyi, and in a nearly allied form, Aeg. heterogeyium. When examining my specimens 

 many naturalists have remarked that they looked as if two different Ammonites had been 

 joined together by mistake. 



When Sowerby figured the specimen collected by Mr. Henley, it was only the body- 

 chamber that was known, which forms the arc of a much larger circle than a corresponding 

 fragment of Aeg. striatum ; this important fact in relation to a specific character was 

 overlooked, and its general likeness led to the belief in their identity ; nor was it until 

 the figured specimen was discovered that the difi'erence became evident which exists 

 between striatum and Henleyi. 



The lobe-line is very imperfectly shown ; the siphonal lobe is as long as the principal 

 lateral, and divides into several small side digitations (fig. 1). The siphonal saddle is not 

 shown. The principal lateral lobe has several small side and two long terminal branches. 

 The lateral saddle is small, and so likewise are the lateral lobe and the auxiliaries. 



The aperture is very large, in consequence of the rapid expansion of the body- 

 chamber (fig. 2) ; it is arched above and flattened at the sides, and is apparently destitute 

 ■of any lateral appendages. 



