INVERTEBRATE 1' A DEONTOLOGY. 451 



wider than the last turn; volutions increasing gradually in size, with con- 

 vexity about three-fourths the dorso-ventral diameter, each turn less than 

 one-tilth embraced by the succeeding outer one; costse simple and closely 

 arranged, in the very young shell, but gradually becoming larger, more dis- 

 tant, and a little thickened at their inner and outer extremities, which latter 

 are slightly curved forward, in examples an inch or so in diameter; periph- 

 eral keel moderately prominent, with the depression on each side shallow. 



Septa not crowded ; siphonal lobe oblong, about one-fourth longer than 

 wide, with two short, narrow, equal, or subequal, nearly simple lateral 

 branches, the two terminal of which are diverging and moderately distant ; 

 first lateral sinus as long and nearly twice as wide as the siphonal lobe, and 

 deeply divided into two nearly or quite equal parts, with merely sinuous and 

 obtusely digitate margins ; first lateral lobe slightly longer than the siphonal, 

 and of about the same breadth, with some five or six spreading, unequal 

 digitations at the posterior end, the middle two of which sometimes become 

 more prominent, so as to give a slightly bifid appearance to the extremity ; 

 second lateral sinus short, or scarcely more than half as long on the inner 

 side as the first, subquadrate in form, with shallow marginal sinuosities; the 

 mesial very shallow indentation, causing a faint tendency to a bilobate outline 

 at the anterior extremity ; second lateral lobe very small, or even less than 

 the auxiliary lobe of the first lateral sinus, about twice as long as wide, 

 narrower, and truncated at its posterior end, with a few very shallow sinuos- 

 ities along its lateral margins ; third lateral sinus hardly half as long or wide 

 as the second, and merely faintly bilobate at the .end; third lateral lobe a 

 little oblique, simple, and smaller than one of the principal terminal digita- 

 tions of the first lateral lobe. 



Greatest diameter, 1.10 inches; convexity, about 0.26 inch. 



The little specimen from which the foregoing description was made 

 out is doubtless a young shell. If not, it would not properly go into the 

 group Mortoniceras, as its costse and periphery are without nodes; the former 

 being also more curved forward at their outer ends than is usual in the 

 typical species of the same. Its costse, however, on the outer volution, show 

 rather a distinct thickening at both extremities, and it is very probable that 

 in larger adult individuals distinct nodes are developed. The lobes and 

 sinuses of the septa would doubtless at that size be found more deeply 

 divided and branched. 



