Summit Structure of Pentremites. 493 



division of the ray in which it occurred, with the corre- 

 sponding ray or division in the same or other specimens. 



Such losses are most common in the more globose 

 species, and sometimes cause considerable difficulty in 

 determining the normal number of arms to each ray, the 

 variation of which is often the principal difl'erence in 

 closely allied species, at least so far as the usual condi- 

 tions of their preservation will allow us to determine. 



No instances have been observed of the replacement of 

 the brachial appendages when amputation occurred in 

 close proximity to the body, or principal rays, which is 

 taken to indicate the probability that the power to repro- 

 duce lost parts did not extend to the principal rays and 

 the body ; yet their power of sustaining and recovering 

 from severe injuries is most remarkable. It is most prob- 

 able, however, that those instances of the want of an en- 

 tire side, or ray, are the result of embryonic deficiency, or 

 possibly occurred while the animal was young, and in a 

 vigorous stage of growth. 



8. A specimen of Actinocrinus rusticus, Hall, has an 

 extra plate developed between two of its first radials, 

 making the interradial series in which it occurs greater 

 than the others, and equal to the anal series, and present- 

 ing a series of seven plates of about equal size, surround- 

 ing and adjoining the basal plates. This is probably not 

 the result of an injury, but is given as a probable case of 

 excessive embryonic development, and in contrast with 

 the supposed embryonic deficiencies in some of the pre- 

 vious cases. 



9. Different parts of the body and column of crinoids 

 are frequently found, showing evidences of having suffered 

 a disease of the exterior surface. The genus Cyathocrinus 

 has been found thus affected, but Actinocrinus and Platy- 

 crinus seem to have suffered most, especially the latter. 



The characters of the traces of this malady are essen- 



