402 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES 



compressed toward the larger extremity of specimens three inches or so in 

 length. At a somewhat larger size, say an inch in diameter, the compression 

 becomes more marked ; and at one and a half to about two inches, it is pro- 

 portionally most strongly developed. Beyond this, the proportional convexity 

 increases again, until it is about as great as usual in B. ovatus, though the 

 greatest convexity is near the antisiphonal side, from which the lateral sur- 

 faces converge with only moderate convexity to the very narrowly-rounded 

 periphery, so that the section of the non-septate portion of these larger 

 adult examples assumes an ovate-subtrigonal outline. 



These different forms of the shell at different stages of its growth are 

 illustrated by the following outline cuts; the outer line showing the section 

 of an adult shell; and the next within, its section at medium size, where it 

 is most compressed ; while the others show its more oval, or less compressed 

 section at smaller sizes. 



Dr. Morton in this country, and M. d'Orbigny and some others in Europe, 

 have regarded this shell as only a variety of B. ovatus, Say ; while Dr. Bink- 

 horst, in his Monogr. des Gasterop., et Cephalop. de Craie Sup. du Lim- 

 bourg, page 41, on the contrary, cites it as a synonym of B. Fanjasi, Lam- 

 arck. In the latter opinion, however, I cannot by any means concur. In 

 regard to its relations to B. ovatus, however, there are some reasons for 

 doubt. The distinctions between this shell and B. ovatus, as here defined 

 and illustrated, hold good in a very great majority of instances, through 

 large collections, including many hundreds of specimens of all sizes. Still, 

 however, we meet with an occasional fragment, with the compressed form 

 and slender and sharply-digitate lobes of B. ovatus, having the little sinus 

 between the two terminal divisions of its first lateral lobe so deep as to 

 obliterate entirely the stem-like part usually supporting them, and thus to 

 give, with the two adjacent lateral branches, a quadripartite appearance, as 

 in B. ovatus, instead of the usual tripartite arrangement with the middle 

 division bifid, as we nearly always see in B. compressus. Again, on the other 

 hand, we very rarely see fragments with the ovate form of B. ovatus, and 

 the slender, deeply divided, aud very sharply digitate lobes of B. compressus, 

 showing the terminal divisions of the first lateral lobe, very nearly as in B. 

 compressus, though supported by a shorter stem-like portion of the body of 

 the lobe than we nearly always see in the latter. 



