OF CONCHOLOGY. 123 



spines are nearly as large as the primary ones. In order to 

 appreciate the value of this character, it will be advisable to pass 

 all the species in review. 



1st. P. lamhis and its nearly related congeners, P. crocata, 

 and P. hryonia, have a perfectly entire lip between the primary 

 digitations.* (Digitations S. 1, A. 1+1+1. M. 1. P. l.)^6. 



2d. P. Scorpio and P. pseudoscorpio are provided with small 

 dentiform processes, caused by the expansion of the larger re- 

 volving striae at their ends. 



In P. pseudoscorpio there are two, one between the pre-angu- 

 lar and medial spines, and another between the medial and post- 

 sinual. Digitations (S. 1. A. 1+1+1. (r). M 1. (r). P. 1.)= 

 6+2 r. 



In P. scorpius, however, not only do the corresponding strife 

 and those on each side thus terminate, but those diverging 

 and running out on the sides of the digitations expand and are 

 abruptly truncated, thus producing the nodose appearance of the 

 digitations of that species so characteristic of it. Digitations 

 (S. 1. (r) A._ 1 (r) 1 (r) 1. (r.r). M. 1. (r.r). P. 1.)= 6+vx.-f 



3d. P. millipeda, P. elongata, and P. violacea exhibit, at the 

 margin of the lip, in place of dentiform processes like those of 

 P. scorpius, more or less developed and channelled digitations, 

 and the fascioles corresponding to them are generally developed 

 and almost as well defined as the primary ones. In P. milUpeda, 

 however, they are sometimes no more defined than in P. lambis. 



In P. eloyigata three angular spines are alone developed, (at 

 least generally,) as in P. lambis and P. scorpio and their allies. 

 Digitations (S. 1. A. 1+1+1. (I.) M. 1. (I.) P. l.)z=8. 



In P. millipeda an intercalary digitation is interposed 

 between the sutural and post-angular.| Digitations (S. 1. (I.) 

 A. 1+1+1. (I.) M. 1. (I.) P. l.)=9. 



* Reference is had only to the margin behind the sinus in all cases. 



t In this connection it may be remarked that a specimen of P. scorpius, 

 in the collection of the Smithsonian Institution, exhibits intercahiry 

 spines like P. millipeda ; one between the lower angular and median, and 

 another between the latter and postsinual ; the lip had been broken after 

 having commenced to send forth its spines ; the primary spines are thrown 

 farther out from the lip, and the median one is smaller than the secon- 

 daries. The characteristic cedematous condition of the spines has not 

 been developed, and the specimen was referred by Dr. P. P. Carpenter 

 to '■ P. pseudoscorpio," but the left basal lobe of the posterior spine, the 

 elevated white ridge below the hollow of the dorsal tubercle, and the 

 armed lip (features characteristic of S. scorpius, but hitherto unnoticed) 

 unequivocally point out the true relations. 



X The supernumerary digitation is sometimes undeveloped, as in P. 

 elongata ; there is, it may be added, no connection between such atrophy 

 and that of the intercalary fascioles. 



