ACANTHONAUTILUS BISPINOSUS. 119 



whose flatness is nearly on the same plane as the adjacent part of the surface of 

 the peripheral area, are greatly expanded at the base, but become rapidly narrow 

 towards their distal end, their basal portion merging with a slight concavity in the 

 general surface of the ventral part of the shell on the outer side, while on the 

 inner side they are continuous with the umbilical margin. The spines are met 

 anteriorly by the border of the aperture, posteriorly by the rim of the umbilicus, 

 from which they proceed on that side. They thus make here a curve on their 

 inner edge, which is directed outwards and slightly downwards at the same 

 time. The direction of the spines in relation to the longitudinal axis of the 

 shell is nearly that of a right angle, with a slight curvature of the flattened aspect 

 inclining away from the periphery and towards the umbilicus. 



Both of the spines are imperfect, one having been broken off near the base, 

 the other wanting a portion of the extremity. The former is seen to begin with 

 two diverging folds of the shell, originating as above described. These make an 

 angle, as seen in the section of the broken stump, the blunt apex of the angle 

 being formed by the rim, already described as an extension of that of the 

 umbilicus. The two folds rapidly approach each other, till, as the more perfect 

 spine shows, they run nearly parallel, forming a flattened tube up to the end of 

 the spine, as far as it is preserved. The flattening here referred to may be partly 

 due to pressure in the rock; but, on the other hand, the pinched appearance of 

 the two folds of the shell near the base of the spine, where there is no evidence 

 whatever of crushing, is already so marked as to contradict the supposition of 

 flattening by any such accidental agency. 



The outline of the aperture, which can be distinctly traced, is curved gently 

 forward on either side, and forms in the centre a broad and shallow hyponomic 

 sinus. 



The sutures, of which only five are visible, are moderately distant, the last two 

 somewhat closer together than the rest. The septa have been completely 

 destroyed by the deposition of crystalline calcite in the chambers, the removal of 

 a great part of this calcite disclosing no traces of them ; nor were any remains of 

 the siphuncle met with. 



The distances between the septa which are exposed to view, measured from 

 the posterior to the anterior one, are 14 mm., 17 mm., 13 mm., and 12 mm., the 

 last one measured being probably next to the body-chamber. 



The test is quite smooth, only slightly marked lines of growth occurring on 

 the body-chamber. 



