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the sutural one of P. lambis, P. pseudoscorpio, P. elongata, 

 P. violacea, etc. 



The angular digitation is homologous with the (median) angu- 

 lar of P. lambis, &c. 



The lobal digitation has no developed homologue in front of 

 the inflected lobe of P. Ia7nhis or its associates, although the 

 fasciole may be distinctly developed. 



Of course, no doubt can exist as to the homologies of the si- 

 phoniferous canal, however much the lobal digitation of the one 

 and the anterior one of the other may correspond in apparent 

 position or curvature, and the difference in direction of the si- 

 phonal canal, and its simple trend towards the left in P. chiragra 

 is therefore to be regarded in some manner as the reflection of 

 the development of the post-lobal digitation and formative 

 element in that type. 



The relations and diff'erences between the armature of the two 

 groups thus distinguished, may be expressed in a concise man- 

 ner by the following formula : 



2. Secondary spines. 



While in most of the Lamarckian Pteroceras, the primary 

 spines enumerated are the only ones developed, in one of the 

 groups there are species which possess, in addition, smaller 

 spines interposed between, and more rarely, such intercalary 



* Patholoo:y, which has furnished such aid to Physiolog:y and Anatoniy, 

 may be invoked with advantage iu Conchology. A malformed specimen 

 of P. lambis ei:hibits a peculiar arrangement of the sutural digitations 

 and perhaps furnishes a hint as to the possible manner of diver- 

 gence of the two types so distinct at the present day. The last whorl, at 

 its commencement, is rapidly deflected and thence regularly continued 

 much below the suture, but trends upward as usual towards the matured 

 lip ; instead of a single digitation concurrent with the spire, there is a 

 primary malformed one, which trends backwards like the subsutural of 

 P. chiragra, and another diverging from and crossing the spire towards 

 the left, as the sutural in P. chiragra. An excessive deposit of callus has 

 obliterated the canals at the base. 



