AEGOCERAS CARUSENSE. 373 



is only a fragment of a whorl, and belongs to the Museum of the Irish Survey, Dublin. 

 It was collected by the officers of the Irish Geological Survey, and was obtained from the 

 Aeg. Jamesoni-hed of the Middle Lias at Carncastle. 



This Ammonite is noted in General Portlock's * Report on Londonderry,' p. 133, 

 under the name Ammonites radians, Reinecke, and was found in a highly calcareous bed, 

 approaching to impure grey limestone, at Carncastle, Ballygalley Head, County Antrim. 



Aegoceras Carusense, d' Orhigny. PI. L, figs. 9, 10. 



Ammonites Carusensis, d'Orbigny. Paleont. Franc^. Terr. Jurass., t. i, p. 284, pi. 84, 



figs. 3—6, 1842. 



Diagnosis. — Shell compressed, discoidal ; whorls narrow, numerous, and slightly 

 involute ; sides depressed, and encircled by sharp, straight annular ribs ; siphonal area 

 convex, and crossed by transverse ribs ; umbilicus widely open ; aperture round. 



Dimensions. — 35 millimetres; width of the umbilicus 20 millimetres; height of 

 aperture 4 milhmetres ; width 4 millimetres. 



Description. — This elegant little Ammonite was collected in great numbers many 

 years ago from the Aeg. Jamesoni beds near Cheltenham, associated with the young shells 

 olAeg. densinodum. I was long under the impression that it might be the young form of 

 a larger Ammonite, still as I have never met with one larger than the figured specimen, 

 which possesses its body-chamber, I have come to the conclusion that it is the complete 

 Ammonite. 



The shell is suborbicular and compressed ; the whorls are very slightly involute, and 

 often distorted, so that this Ammonite has been mistaken for a Turrilite. The whorls 

 are rounded on the sides, and encircled by twenty-nine sharp, straight, annular ribs, 

 which bend gently backwards where they cross the area ; sometimes they are interrupted 

 in their transit, but oftener they pass entire. This shell appears to undergo no change 

 in the different phases of its life. The spire is formed of seven narrow, depressed whorls, 

 imiformly ornamented with regular, straight, sharp ribs, and narrow, concave spaces 

 between. The aperture is round and slightly compressed at the sides. 



The lobe-line is very simple. The siphonal lobe is longer and wider than the 

 principal lateral, with simple digitations on its margin. The siphonal saddle is twice 

 the size of the principal lateral lobe, with four simple leaves at its termination. The 

 principal lateral lobe is very small, with a few digits on its sides. The lateral saddle is 

 large, ending in three irregular foliations. The lateral lobe is very short, and ends in 

 three digits, and the auxiliary lobe has only a single point. 



Affinities and Differences. — It resembles the young forms of Aeg. densinodum, which 



