belonging to the Class Palliobranchiata. 85 



two outwardly-inclined plates extending from the socket-walls to 

 the centre, a distance exceeding, by one-fourth of their length, 

 the anterior extremity of the arch. Both plates are attached 

 to the inside or floor of the valve, at a little distance from each 

 other, nearly their entire length, gradually increasing in height 

 and becoming more divaricated as they advance. Looking down 

 upon the plates, their posterior half is seen at first, that is, com- 

 mencing from the floor of the valve, leaning outward, then to turn 

 inward, and again to turn outward ; this brings their superior 

 margin nearly in contact with the postero-lateral margin of the 

 valve to which they are attached : their anterior half is simply 

 inclined outward at first, and then inward ; the difference being 

 caused by the absence of the superior eflected portion, which, de- 

 creasing in depth somewhat rapidly in its progress, is not carried 

 beyond the middle of the plate : in Pentamerus Knightii the su- 

 perior eflection is carried much further forward, and it appears to 

 be the same in P. conchidium. 



Although there is considerable dissimilarity between Penta- 

 merus and other palliobranchiate genera, yet I cannot agree to 

 the amount of difference contended for by M. Verneuil, who 

 recognises little or no identity between the parts composing the 

 internal apparatus of the former, and those entering into the com- 

 position of its homologue in the latter*. 



In the first place let us consider the arch of Pentamerus. The 

 position of the plates composing this arch, relatively to the delti- 

 dium, and their subserviency to articulation, place beyond doubt 

 their strict identity with the condyle plates of other Palliobranchs. 

 This view was first advanced by Von Buch, from an examination 

 of Pentamerus conchidium f. In Productus, &c. the condyle 

 plates are never seen; in Terebratula they are only partially 

 present; while in Spirifer, Atrypa, Hypothyris and Orthis%, 

 they are rarely absent. In those shells which are provided with 

 them, the position of the condyle plates relatively to each other 

 is often very different : in many Orthises, Atrypas, Hypothyrises 

 and certain Terebratulas (T. elongata and T. hastata), they vary 

 slightly from the perpendicular ; in certain Orthises ( O. eximia, 

 O. crenistria, &c), and most of the Spirifers, they strongly in- 

 cline towards each other superiorly, but without coming in con- 

 tact ; in Spirifer heteroclitus, Or this adscendens, Uncites Gryphns§, 



* Geology of Russia, vol. ii. pp. 107, 108 and 109. 



^ Ueber Delthyris, &c. 



X The condyle plates are rudimentary in Orthis senilis, 0. Wangenheimi, 

 &c. The peculiar twist of the umbone in O. senilis, &c. is probably owing 

 to the absence of the condyle plates. 



§ This singular shell has the condyle plates forming a remarkably flat- 



