I N VERTEBRATE PAL^ONTOL< >GT. 401 



Septa usually crowded, and with lobes and sinuses deeply divided into 

 slender branches ; siphonal lobe nearly twice as wide as long, and provided 

 with two widely-separated, tripartite and digitate terminal branches, and one 

 smaller digitate lateral branch on each side; first lateral sinus as long as the 

 siphonal lobe, but not much more than halt" as wide, and very deeply divided 

 at the end into two equal tripartite and digitate branches, with spreading 

 subdivisions; first lateral lobe longer than the siphonal lobe, and about half 

 as wide, with two small, parallel, sharply digitate, termimal brandies, stand- 

 ing, as it were, on a stem formed by the very narrow body, which also sup- 

 ports on each side two opposite, sharply digitate lateral branches, one pair of 

 which shows more or less tendency to tripart division; second lateral sinus 

 usually a little longer, but otherwise very similar to the first; second lateral 

 lobe shorter and broader than the first, and provided with two equal tripartite 

 and digitate, spreading terminal branches, with much smaller, irregular, lat- 

 eral branchlets; third lateral sinus usually not larger than one of the main 

 terminal branches of the others, and deeply bifurcated at the end, the divis- 

 ions being more or less subdivided, or merely digitate ; antisiphonal lobe 

 generally only about half as long as the second lateral, and much narrower, 

 with two to four very small lateral branches, or mere digitations on each side, 

 and one small, tridentate, terminal division. 



As with B. ovatus, this species is only known in the condition of broken 

 specimens, from which accurate measurements of the entire shell cannot be 

 given. It evidently attained as large a size as B. ovatus ; some of the larger 

 fragments of the septate part measuring about 3.50 inches in their greater 

 diameter, by 2.20 inches in their smaller diameter. 



This shell, as may be seen from the illustrations and descriptions, is 

 rather nearly related to B. ovatus, but in most cases may be distinguished by 

 its decidedly more compressed form in medium-sized specimens, and differ- 

 ences in the details of the septa. These differences are most strongly marked 

 in the form of the first lateral lobe, which nearly always differs from that of 

 B. ovatus, not only in having a more slender body, and like the other lobes 

 more sharply digitate branches, but in also having its two terminal branches 

 smaller, and standing as it were on a narrow stem. 



An extensive series of specimens of this shell shows that the young is 

 more rapidly tapering than at any later stage of its growth, and that its sec- 

 tion is then proportionally less compressed. Soon, however, it became, more 

 .51 h 



