260 THE LIAS AMMONITES. 



Genus Toxoceras, d' Orb. — Shell conical, subcylindrical, or compressed, symmetrical, 

 elongated, more or less arched, but never forming a spiral. Sides of the shell ornamented 

 with encircling ribs ; in some forms stronger ribs are developed at intervals. Or the 

 sculpture consists of two rows of tubercles which grow upon each side of the large ribs, and 

 two rows of larger tubercles on the borders of the siphon al area, as shown in Toxoceras 

 Honnoratianum (tig. 179). The mouth is round, oval, or compressed, with a prominent 

 internal border, and the large ribs on the sides and ventral surfaces represent the different 

 stages of growth of this bent cone. The lobe-line is very much ramified ; the siphonal 

 lobe symmetrical, the stem short, and the bifurcate branches long ; the superior lateral 

 very large and composed of nearly symmetrical branches ; the lower lateral is small, the 

 columellar lobe has considerable dimensions, and is nearly symmetrical in its structure. 

 The affinities of this genus, regarding the structure of its lobes, is with the preceding genus 

 Crioceras ; and, like it, it is found in the lower stages of the Cretaceous rocks ; all the 

 larger specimens are obtained from the lower and upper Neocomian strata. 



Fig. 179. — Toxoceras Honnoratianum, d'Orh. Fig. 180. — Aspidoceras longiipinum, Sow. 



Genus Aspidocekas, Ziifel. — The form of the shell in this genus is very variable. 

 Sometimes it is flat and discoidal with a wide umbilicus, or large, inflated, and 

 highly involute. The siphonal or ventral area is rounded or flattened, the sides are 

 adorned with fine ribs, and the sculpture consists of two rows of tubercles developed at 

 intervals among the finer folds of the shell in its early age, which seem to disappear 

 or are undeveloped in later years. The lobe-line ramifications are simple in Aspid. 

 perarmatum, Sow. The siphonal lobe is large with symmetrical branches, the principal 

 lateral is large and composed of numerous unequal parts, and the lower lateral is much 

 smaller. The body of the lobes and saddles is broad, and the lobes are much slit up 

 into branches. Aspid. longispinum. Sow. (fig. 180), from the Oxfordian of Weymouth, 

 has a thick, discoidal, smooth shell, with two concentric rows of short spines growing upon 

 the sides of the whorls, which are few and more than one half involute. This forms a 

 very good type of the group. The large section of the Perarmati, with a double row of 

 tubercles on the sides of the whorls, appears to want auxiliary lobes. In the group 

 which has Aspid. Altenense, d'Orb., from the Corallian, for its type, one row of tubercles 



