354 THE LIAS AMMONITHS. 



extends aloug the middle of the siphonal ai'ea. The large specimen (PI. LI, figs. 1, 2) from 

 Mungar shows a very remarkable development of the ribs on the sides as well as on the 

 siphonal area ; these aged forms may, therefore, represent Jamesoni latum, Quenst. In the 

 middle period the fossils figured in PI. XI, figs. 4 — 6, and in PI. LI, fig^. 3, 4, maybe 

 taken as representing Ae^. Jamesoni an(justatum, Quenst. ; the sides of the whorls are 

 convex and compressed, and the ribs slender and very numerous ; in one specimen there are 

 more than seventy ribs on the last whorl. Sometimes the ribs are fewer in number and 

 stronger, as in the fig. 3, in which the forty-two ribs are sharp and prominent with 

 wider intermediate valleys, presenting a strong contrast to fig. 4 on the same PI. LI. 



The Middle Lias in Robin Hood's Bay, Yorkshire, yields another variety of Aeg. 

 Jamesoni, which has been described and figured as a new species by Professor Blake 

 under the name of Aeg. sagittarium. I have long known this variety, which has roundish 

 whorls nearly one-third involute, and large, sharp, sigmoidal ribs, about twenty-four 

 in a whorl, with deep concave valleys between the ribs, which arch forward across the 

 siphonal area. I have figured the young shell of this variety in PL LII, figs. 4, 5, and 

 the large adult shell collected by the Rev. Mr. CrosSj F.G.S., from the Jamesoni-Lias 

 in Robin Hood's Bay. Por further details consult the article on Aegoceras sagittarium. 



The lobe-line is very well seen in several of my specimens. The siphonal lobe is shorter 

 and wider than the principal lateral, and ornamented on each side with three ramified 

 branches. The siphonal saddle, much wider than the principal lateral lobe, divides 

 into two nearly equal-sized foliations. The principal lateral lobe is long and much com- 

 plicated, and divides into three ramified branches. The lateral saddle divides into two 

 unequal parts, of which the internal is the most developed. The lateral lobe is slender 

 and narrow, and consists of a single stem with lateral digitations ; and the auxiliary 

 saddle is narrow with a few irregular folioles. 



The morphology of Aeg. Jamesoni is extremely interesting. Although only fragments 

 of large whorls have been collected, I have found many very perfect specimens of the brood 

 of this species in the Middle Lias of Leckhampton. A good example is figured, PL LI, figs. 

 5, 6 ; in this stage it is Ammonites Bronni, Romer. The shell is ribbed on the sides in the 

 second turn of the spire ; and in the fourth turn the sharp ribs acquire small tubercles by 

 the side of the area, fig. 5 ; and as they arch across that space a well marked carina is 

 observed on the mesial line, fig. 6. With the growth of the shell the tubercles decrease in 

 size, and finally disappear in middle age ; the area, becoming narrow and convex, gradually 

 assumes the form delineated in PL LI^ figs. 3 and 4, as the typical forms of middle life. 



Affinities and Differences. — I have long collected the young of this Ammonite in the 

 Middle-Lias brick-yards near Cheltenham ; in this state it is Am. Bronni, Romer ; another 

 form is Am. Itegnardi, d'Orb., and another variety Am. venustulus, Dumortier. It is a 

 very variable fossil, and has been a fertile field for species-mongers. I have specimens from 

 Robin Hood's Bay identical with the Pabba examples ; but the large, ribbed, figured 

 specimen, with its undulated sides and waved area, is a very rare variety of this shell; 



