CRICK : ON PLEUR0NAUTILU8 PULCHEB, N-SP. 19 



and neanic stages, as well as the extent of the central vacuity ; the 

 ananepionic shell is seen abutting against the succeeding whorl, but 

 the terminal view of this substage is obstructed by matrix ; at the 

 end of the first whorl the shell has a diameter of 9*5 mm. 



The larger example consists of two whorls ; the inner whorl is less 

 perfectly preserved than in the smaller specimen, and only portions of 

 the test are present. There are nineteen or twenty costiie in the outer 

 whorl. The body-chamber occupies nearly one-half of the last whorl ; 

 the penultimate and antepenultimate chambers are each 2*5 mm. deep 

 at the centi'e of the periphery, the whorl here having an altitude of 

 6'25 mm. ; the last chamber is somewhat shallower than the penulti- 

 mate, a fact which, notwithstanding the comparatively small size of 

 the specimen, is usually regarded as a sign of maturity. On the 

 whole this shell is rather stouter than the example selected as the 

 type, but this may be due either to individual variation, or, as is more 

 probable, to sexual difference. 



The dimensions of the specimens arranged in the order of their 

 respective greatest diameters are given in the following table ; 

 dimensions at other diameters being added for the sake of com- 

 parison, (i) is the smaller example in Dr. Wheelton Hind's 

 collection ; (ii) the smaller specimen in the British Museum col- 

 lection [No. C. 213]; (hi) Dr. Wheelton Hind's larger example; 

 (iv) the larger specimen (the type) in the British Museum collection 

 [No. C. 5277]. Tlie measurements are in millimetres. 



Table of Dimensions. 



I 



Diameter of shell 17'5 



Radias of shell * 10 



Width of umbilicus ... 6*5 

 Height of outer whorl . 6 

 Thickness of outer whorl 8'5 6 '5 



With regard to the systematic position of the species, it seems to 

 belong to Mojsisovics' genus Pleurona^dilus ' (emended by Foord -) 

 except that in that genus, according to Foord, the siphuncle is a little 

 below the centre, whereas in the present species it is a little above 

 the centre in all stages of growth. The range of the genus is 

 considered to be from the Devonian, where it is represented by 

 P. subtuherculatus, G. & F. Sandberger,^ to the Trias, from which 

 formation several forms have been described by Mojsisovics and others. 



I.e., a line drawn from the centre of the coil to the periphery of the shell. 



The last fourth of the whorl being imperfect both laterally and ventrally, these 



measurements are only approximate ; they can be best taken where the diameter 



is 215 mm. 

 Test well preserved ou each side. 

 E. V. Mojsisovics : " Die Cephalopoden der Mediterranen Triasprovinz " (Abhandl. 



der k.-k. geol. Eeichsanst , Band x), 1882, p. 273. 

 A. H. Foord: Cat. Foss. Ceph. British Museum, pt ii (1891), p. 1.34. 

 G. & F. Sandberger: " Verstein. rhein. Schichtensyst. Nassau," 1854-56, p. 133, 



pi. xii, tigs. Za-e [Nautilus tuberculatum). 



