NAUTILUS TRUNCATUS. 35 



leaving no part of the inner whorls exposed. The peripheral area in the adult 

 has a well-marked change of aspect, so that its edge is almost angular, but 

 this becomes less so as it is traced backwards towards the origin, near which it 

 becomes almost round. The sutures have a more or less marked backward curve 

 on the peripheral area; they rise to their most forward point on the margin. 

 On the lateral area they form a long backward curve, rising again near the 

 umbilicus, into which they curve backwards again. (Sowcrby is wrong on tin's 

 point, even for his type specimen.) To produce this form of suture the front 

 surface of the septum must be distally concave from side to side as well as in a 

 radial direction, except towards the umbilicus, where it must change in the latter 

 direction to convex. Other specimens show that the siphuncle was about one third 

 the height of the aperture towards the periphery, and that the shell was thick and 

 had no ornament beyond lines of growth. 



In a large specimen of 250 mm. diameter found at S. Cerny, the periphery had 

 become concave, like N. giganteus, and the backward bend of the sutures had 

 consequently become well marked. On the other hand, when the species is 

 smaller the periphery becomes more and more round, as in the case figured in 

 fig. 2. It scarcely looks the same species, but the change may be traced almost 

 continuously. The following are some measured examples : 





Specimens. 



Diam. in mm. 



iuu oi iran.svei 

 diam. 



Ratio of thickness. 



1. 



Bedford, B. M., No. C. 5077 . 



350 



76 



. . 62 (?) 



2. 



Bedford, B. M., No. C. 7297 . 



116 



74 



54 



3. 



Scarborough, Sedgwick Mus. 



160 



75 



60 



4. 



Kushden, B. M., No. C. 3473 . 



61 



73 



54 



Relations. — This species does not appear to be the same as the " JV. truncatus, 

 Sow.," figured by D'Orbigny (' Terr. Jur.,' pi. xxix) as from the Upper Lias of 

 Dijon, which has a slower rate of increase and a different form of septum. The 

 N. baberi of Morris and Lycett, which, as those authors say, is closely allied to it, 

 and to which, as it would appear, several specimens of it have been referred, have 

 no such depression in the umbilical region, nor the same style of suture. It 

 develops also other peculiarities in the specimens. N. gigan tens, however, seems to be 

 a development of it. This is a widely spread form, ranging, according to its author 

 (D'Orbigny, 'Ann. Sci. Nat.,' vol. 5, pi. vi, fig. 3, 1825, and ' Terr. Jur.,' pi. xxxvi), 

 through the whole of the Oxfordian, therein including the Callovian. It is also 

 a large form, as its name implies, and is said to have the " back round " in youth, 

 but the periphery is concave. In this it agrees with our S. Cerny specimen, 

 and there is in the British Museum (No. 325t> :> >), from the Oxfordian of Wilts, a 

 specimen with a similar concave periphery in the outer whorl, which, in the inner 

 whorl, imbedded in place, is scarcely even flattened. D'Orbigny's figure, however, 

 represents a more open umbilicus, but this may be due to a more vigorous growth, 

 the specimen being still septate at a diameter of 600 mm. 



