imperfectly-defined Species of Jurassic Nautili. 269 



large ; whorls flattened both on the sides and on the periphery, 

 the greatest thickness being at the umbilicus. The sides of 

 the latter are steep and the borders subangular, the test being 

 here very thick. The septa, of which there are about thir- 

 teen in an entire whorl, form a strongly marked sigtnoidal 

 curve upon the sides of the shell, but are only very slightly 

 sinuous on the periphery. The siphuncle is situated a little 

 below the centre. The test is covered with numerous, thread- 

 like, longitudinal ridges, two or three in the space of 1 line, 

 more approximate on the periphery than on the sides, as is 

 usually the case with such ornaments. The ridges become 

 partly obsolete when the shell has attained a certain dia- 

 meter, say between 4 and 5 inches. Fine lines of growth are 

 seen where the shell is well preserved, especially in the region 

 of the umbilicus. 



It should be added that the interior of the umbilicus is 

 ornamented with fine, radiating, flexuous ridges directed 

 forward ; these are crossed by spiral ridges somewhat widely 

 spaced. 



Remarks. This species is distinguished from N ornatus (to 

 be subsequently described), which appears to be its nearest 

 ally, by its more compressed form, more angular umbilicus, 

 and finer sculpture. 



Horizon. Upper Lias. 



Locality. Floore, Northamptonshire. 



3. Nautilus terebratuSj Dumortier. 

 Fiar. 3. 



Nautilus terebratus. — a, lateral view, showing the raised border of the 

 umbilicus; b, front view. Drawn from a specimen in the British 

 Museum (no. C. 3096). Two thirds natural size. Except in well- 

 preserved specimens the longitudinal ornaments are barely visible ; 

 we have therefore given a separate figure of them (fig. 4). 



