of British Mollusca. 461 



hooked acetabula, the shell has its phragmocone largely deve- 

 loped, while the mucro, which is so conspicuous in typical 

 Belemnites, is here almost evanescent. In the Solenhofen 

 beds of Bavaria, which are supposed to be of nearly similar 

 horizon to the Kimmeridge Clay, the genus AcantJioteutJiia 

 appears to be a decided Chondrophore, with hooked acetabula, 

 and in Conoteuthis Dupinianus, d'Orb., from the Gault, we 

 find a shell which shows approach to that of O/nmastrephes, 

 though the arms are unfortunately unknown. 



Belemnites first appears, I believe, in the Jurassic forma- 

 tions. In the Lower Lias it is abundant, and with it is the 

 genus Geoteuthis, which is perhaps a Chondrophore. In the 

 Upper Lias occurs Teuthopsis, which is certainly a Chondro- 

 phore, but the specimens in the British Museum show no 

 appendages to prove whether it is more likely to have been an 

 Oigopsid or Myopsid ; but forms ascribed to the same genus 

 in the Oxford Clay have rather short and broad arms, which 

 show no sign of hooked acetabula, and must belong to an 

 animal allied either to the Ommastrephidffi or the Loliginidas. 

 The Sepiophora first appear in the Kimmeridge Clay in 

 Coccotenthis latipennis, Owen, but the bone only is known ; 

 and there is in the British Museum an unnamed species from 

 the Solenhofen deposits of Bavaria, the bone of which shows 

 a very near approach to the form of the cuttle-bone in existing 

 species of Sepia. 



The origin of the Spirulidae is most obscure. They are 

 possibly derivatives from one of the more simple forms of 

 Ammonite or from such a genus as Spiruli rostra. 



Classification by the shell is very valuable for that purpose 

 with respect to fossil forms, since it is always preserved if any 

 part of the animal is fossilized ; while, on the other hand, the 

 eyes are useless in the investigation of fossil forms, as their 

 structure is rarely (if it is ever) recognizable. But while fully 

 weighing this fact, and holding such classification to be a much 

 truer division according to our present knowledge than that of 

 cl'Orbigny, it appears to me that our insight into the hecto- 

 cotylization in this class has now attained sufficient importance 

 to justify us in employing it as an important factor in classi- 

 fication. Steenstrup, in the paper to which I have already 

 referred, insisted on its value. He wrote : — " The justifica- 

 tion of the mode here adopted of employing the hectocotylized 

 arm as a rule for the natural collocation of the forms lies in 

 its importance for the entire reproduction. It would be incon- 

 ceivable that the various occurrence of this metamorphosis, 

 sometimes in one sometimes in the other pair of arms, some- 

 times on the right and sometimes on the left side, some- 



