20 THE NAUTILUS. 



1*5 to 6 cm. in diameter. An examination of the form of the septa 

 and suture lines showed the forms to belong to the Ammonitidae, 

 and by the examination of an extensive series it was possible to 

 determine the genus and species. 



The shell originates in a s]>iral of two to two and one-half turns, 

 ranging in breadth from 0'8 to 1 mm. thence it extends in a straight 

 line, tangent to the spiral (figure 1) or sometimes slightly reflexed 

 (figure 2). The straight portion of the shell rapidly increases in 

 diameter from 0"38 to 0'40 mm. at the spiral, to about To to 2 mm. 

 at 2 cm. length. Many shells were covered by the nacreous shell 

 substance, some being preserved entire, figure 2, while in others the 

 shell had been dissolved away leaving the suture lines exposed as 

 in figure 1. On breaking away the jDearly exterior of forms like 

 figui'e 2 it was found that the last chamber occupied about one-half 

 the length of the shell. The shell of the outer whorls somewhat 

 envelops the inner so that from the outside view the exact form of 

 the spiral cannot be measured ; it was found, however, to closely 

 approximate the mathematical curve known as the hyperbolic 

 spiral. That the spiral origin of this shell was not smaller than 

 that of allied genera was demonstrated by grinding cross sections of 

 the shell of Scaphites Conradi Morton ; the first two turns of its 

 spiral being 1 mm. in breadth. The siphon in Baculites is excentric 

 and was found to lie near the outer margin of the spiral, being 

 easily seen in the fractured spirals. 



The species was determined from an examination of the form of 

 the sutures which may be traced from the simple form of that of 

 figure 1, through forms of gradually increasing complexity shown 

 in figures 3, 4, 5, and 6, the latter being the typical sutures of the 

 adult of Baculites compressus Say. In figure 5, an individual of 

 6 ram. diameter, the suture of the adult form is already well out- 

 lined, the specific distinction, the two deep sinuses on the right 

 hand, being well marked. 



I have been unable in the literature of the subject to find any 

 reference to this spiral termination of Baculites, and believe the 

 observation to be new. Tliat this spiral termination has not been 

 formerly observed is not strange in view of its small size and frag- 

 ile character, it being probably broken off long before the shell 

 had attained adult size ; and it would only be preserved when as in 

 the present instance the shells were preserved in their immature 

 condition. This observation tends to prove that Baculites origi- 



