32 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



large ossicle, the basal supramarginal plate. It is thought that the 

 ancestors of Hudsonaster had a dorsal disk skeleton made up of a 

 centro-dorsal, a first ring of 5 primordial radials, and a second ring 

 of 10 plates, 5 of which are" the second radials, and 5 interradial 

 pieces, the primordial supramarginals. In Hudsonaster, however, 

 some progression has taken place in the disk in the way of increase 

 of size. Therefore additional ossicles of the secondary skeleton have 

 developed to take up this space, these accessory disk pieces being 

 inserted between the centro-dorsal and the first ring of primordial 

 radials. Such* a development of a small number of accessory pieces 

 here is a hint of future complexity and the rise of the secondary skel- 

 eton, and we shall see how in later and derived genera their number 

 becomes multitudinous and their places of insertion nearly everywhere 

 in the older portions of the skeleton between the columns of ossicles. 



The supramarginals do not margin the animals, but lie inside the 

 inframarginals and axillaries which do margin Hudsonaster and most 

 of the Paleozoic asterids. 



From Hudsonaster to all other progressive asterids of the Phane- 

 rozonia type, the change lies mainly in the increasing number of the 

 ossicles, relative decrease in the size of the plates, introduction of 

 many new series of Accessory pieces, absorption and removal of 

 others, with a marked general tendency to break up the stiff and 

 ponderous inherited skeleton into one of small pieces, thus affording 

 greater flexibility and greater podial strength through the endless 

 duplication of ambulacral parts. These developmental tendencies 

 take place more especially on the dorsal area and are further accent- 

 uated through increase of body cavity, which demands an enlarged 

 skeletal covering. The disk widens, and along with it the proximal 

 parts of the rays, so that interbrachial areas are also affected, until 

 finally in more than one phylum pentagonal asterids result. The 

 interbrachial areas are dorsaUy increased by the insertion of acces- 

 sory pieces between the infra- and supramarginalia, and ventrally 

 by the crowding into these areas of, first, the single axillaries, and 

 then more and more of the oldest inframarginals in pairs, assisted 

 also here by the development of accessory ossicles. 



Origin of the wriggling type of starfish. From Hudsonaster to the 

 other progressive asterids of the Cryptozonia type the evolutionary 

 tendencies are in the same directions, but here even greater flexibility 

 appears to be the main stimulus. Accordingly, the entire dorsal skel- 

 eton tends to break up into small loosely adjoining pieces and finally 

 even into a spicular spinose mesh. In these forms the ossicles of the 

 primitive columns are no longer discernible as such, and this tendency 

 is very apt to be likewise true of the primordial disk plates. On the 

 ventral side the inframarginals are no longer wholly present as such, 

 but may be here mixed up with the dorsal skeletal complex. How- 



