BELEMNITES OP THE LIAS. 



69 



Dimensions. Greatest length observed (including only the beginning of the phragmo- 

 cone), 10 inches ; greatest diameter in the post-alveolar region, less than i inch. 



Proportions. Taking the diameter from back to front at the alveolar apex as 100, 

 the ventral radius is 48, the dorsal 52, the cross diameter from 80 to 90, the axis of the 

 guard about 1400. 



Phragmocone. Incompletely observed j probably straight, with an angle of 

 about 18°. 



Locality. In Upper Lias shale, above the Jet-bed at Salt wick, near Whitby [FUllips). 

 Sandsend, near Whitby {Phillips), and Robin Hood's Bay {Cullen). 



A specimen in the Bristol Museum (b 1, 42), is said to be from Gloucester. It closely 

 resembles, however, Yorkshire specimens, and I have never observed the species in any 

 locality south of Yorkshire. 



Ohservations. By German writers this is usually supposed to be identical with 

 some one of the many forms included under the title of Belemnites acuarius. Quenstedt 

 figures, pi. XXV, figs. 2, 9, 10, specimens from the Upper Lias of Ohmden, which he 

 terms B. acuarius tiibularis ; they are uncompressed, and show a distinct ventral groove, 

 and an undulated alveolar border. D'Orbigny includes P. tiibularis among the many 

 synonyms of P. acuarius with P. longissiiaus. Mill., gracilis, Hehl., lag en csf or mis, 

 Hartmann, longisulcatus, Voltz, tenuis, Miinst., semistriatus, Miinst., gracilis, Romer. 

 He regards the guard as subject to a remarkable extension retrally, at a certain, age, 

 previous to which it does not differ, he says, from that of P. irregularis. The extension is 

 stated to be most frequently hollow, so that the term " tubularis " would be really 

 deserved, and the compression to a flat plate easily explained. When this is not the 

 case, from the filling of the cavity with calcareous matter (not fibrous), it constitutes the 

 so-called Pseudohelus of Blainville. Such is the view of D'Orbigny, who was ready 

 to unite P. irregularis and P. acuarius in one species, the former being females, which 

 preserved always their original obtuseness, and had no retral extension, the latter 

 being males. This remarkable opinion he bases on an examination of the recent Loligo 

 subulata, the males of which have a very long dorsal plate (osselet), the females a 

 short one. I propose to consider this subject in a general discussion, embracing other 

 species; but at present it appears only necessary to say, that there is no ground for 

 admitting the specific affinity of P. irregularis, or any forms like it, with P. tubularis 

 of the Yorkshire coast. 



The length of the unflattened part of this Belemnitic guard is very unequal in 

 different specimens of about the same total length — one example in ray possession gives 

 eight inches for this part, several three inches, one two and a half. The flattened part in 

 one example exceeds six inches. A fine specimen obtained by the Rev. Dr. Pluraptre, at 

 Whitby, and now in the Oxford Museum, is thus measured: total length 12 inches, 



10 



