100 THE NAUTILUS. 



medio excavata, bifida," etc., and from this character the name of the 

 group has been derived. A close examination of the different forms 

 as well as of immature examples leaves no doubt that it is in fact 

 composed of two different lamelhie, the parietalis (inner, deeper) and 

 supraparietalis (outer, or " angular"), almost separate, side by .side 

 in some species {recondita Tapp.-Can.), united to almost a simple 

 one in others (most of P. rupicola Say), comparatively small in P. 

 curvidens Gld. and pentodon Say, as here the siipraparietal is very 

 small or almost obsolete. 



The columellar, equally constant, is generally also somewhat com- 

 plex. The typical inferior and superior jialatal plica3 are always 

 present, though sometimes quite small, and are, as a rule deep seated, 

 never reaching the margin ; in some species one or the other of them 

 is in a peculiar oblique position (P. contrada Say, P. recondiid). 

 Generally there is a " tooth " or short fold at the base, in some 

 species present or absent. Additional dentiform or lamelliform 

 plic?e, sometimes very small, but characteristic, are found in many 

 species; one on the parietal wall, between the " parietal " and the 

 columella, constant (P. armigerella Reinh.) or inconstant (P. curvi- 

 dens Old.), one above the upper palatal (P. armifera Say) one 

 between the two palatals, inconstant (P. pentodon Say, curvidens 

 Gld.) 



As there is no rule without exceptions— and in natural science these 

 " exceptions " are always highly interesting ! — some, or even all, of 

 the typical folds may be absent in species which we have reasons to 

 range under this subgenus (P. corticaria Say, arizonensis W. G. B.). 

 But in general they are remarkably constant throughout the whole 

 group which extends over North, Central and the northern coast of 

 South America, the West Indies and Bermuda, Eastern Asia, and 

 the islands of the Pacific and Indian Seas. Europe has no recent 

 forms ranging in the group ; but there is a fossil one, P. lamellidens 

 from the miocene of Tuchoritz, Bohemia, closely allied to our P. con- 

 tracta Say. 



The species have been ranged under different subgenera, such as 

 Papilla, Leucochila, which neither comprise the whole group, nor 

 are homogeneous in themselves, and which can only gain by the 

 removal of these forms. 



There are several distinct groups of which peculiar characters, the 

 range of distribution and the species will be stated in the following.^ 



1 Conf. Nautilus, VI, p. 4. The species enumerated there will be omitted 

 here; and so will other species which I do not know well enough as to their 

 identity, or their posiiion. 



