FKOM THE CRETACEOUS FORMATION OF NEBRASKA. 401 



branches, the terminal one much the largest and deeply bifurcate, with the divisions 

 smaller than the lateral branches ; terminations sharply digitate. Lateral saddle 

 slightly wider and higher than the dorsal saddle, auxiliary lobe longer and dividing it 

 into two parts corresponding in their details to those of the dorsal saddle. Inferior 

 lateral lobe shorter than the superior lateral lobe, and deeply divided into two unequal 

 branches, that on the ventral side being unequally divided into three, and that on the 

 dorsal side into two parts, all sharply digitate. Ventral saddle one third smaller than 

 the lateral saddle, deeply and somewhat obliquely divided, by a sharply digitate, auxil- 

 iary lobe, into two nearly equal parts, each again less deeply subdivided and having 

 the terminations all obtuse. Ventral lobe narrow, and of the same length as the 

 auxiliary lobe of the dorsal saddle, sharply digitate, the divisions divaricate. 



Angle of the apex (as deduced from measurements of imperfect specimens) about three 

 degrees. The longest diameter of the largest fragment (a septate cast) in our collec- 

 tion is 2.16 inches, and the shortest diameter 1.07 inches; siphuncle of the same indi- 

 vidual, .1 inch. 



In addition to the external differences already mentioned, we may state that the 

 J5. compressiis is never, in our specimens, marked by the strong undulations which 

 characterize the ventral half of the side of B. ovatiis. The internal differences are 

 equally striking and characteristic. The dorsal lobe of B. compressus is proportionally 

 much wider and less deep, and the two branches much more distant and more divergent 

 than in B. ovatiis. The central or siphuncular portion of the dorsal lobe in B. compres- 

 sus presents three small auxiliary lobes, tlie two outer of which are divergent and 

 digitate at their extremities, while outside of these, and between them and the 

 main branches of the dorsal lobe, are one or two subordinate digitations; while in 

 B. ovatus the same region is marked by only two short and parallel extensions with a 

 minute point between them, or over the siphuncle, and some undulations on each side. 



In B. compressus the lobes and auxiliary lobes are all more narrow, longer, and, 

 together with all their subdivisions, much more acute, than those of B. ovatus. The 

 superior lateral lobe in the two species likewise presents a striking difference. In 

 B. compressus it is divided into three nearly equal branches, the terminal one of which 

 is again deeply bifurcate ; while in B. ovatus this lobe, in consequence of the greater 

 depth of the terminal sinus, is divided into four nearly equal branches, the two ter- 

 minal ones being bifurcated by a small sinus. Similar differences are noticed in the 

 inferior lateral lobes of the two species. Another difference may be observed in the 

 relative size of the siphuncles, that of B. ovatus being proportionally longer. 



VOL. V. NEW SERIES. 54 



