50 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



axillaries occur in many other forms where they are, however, pushed 

 orally into the skeleton of the interbrachial areas. Single, not double, 

 axillaries, therefore, make the simplest and smallest type of inter- 

 brachial areas. Further complexity and enlargement of these areas 

 are produced by crowding into them orally, first the axillaries and 

 then one or more pairs of proximal inframarginals. 



In Hudsonaster narrawayi one sees tiny single plates proximal to 

 each axillary. These are clearly of no marked morphologic value and 

 are to be interpreted as accessory plates. Similar single ossicles are 

 also seen in genera with large interbrachial areas, as in Promopalseaster. 

 They are regarded as space-fillers and without ancestral significance. 



Interbrachial areas. The simplest type of interbrachial area is that 

 just described. Increase in size and complexity of interbrachial struc- 

 ture is a natural result from an enlarged body cavity. The first change 

 in this direction is seen in Mesopaldeaster, Spaniaster, Miomaster, and 

 Devonaster, where the axillaries are crowded orally and surrounded 

 distally by a pair of adjoining basal inframarginals, and proximally by 

 a variable number of adambulacrals. The same development is seen 

 but hi somewhat modified form in the cryptozonian genera Urasterella 

 (U. ulrichi and U. girvanensis) and CaUiasterella. Here the axillaries 

 are present only in the young, but may be retained to maturity; in 

 either case they may or may not be margined distally by basal infra- 

 marginals. As the inframarginals in these forms are vanishing (pri- 

 mitive Cryptozonia), we should expect to find related genera devoid 

 of even the axillaries, and such is the case in Stenaster and Tetraster. 



The next step in the enlargement of the interbrachial areas consists 

 of the inward crowding of pairs of basal inframarginals, one or more 

 at a time. In Trimeraster these areas have the single axillaries and a 

 pair of basal inframarginals, with the second pair of proximal infra- 

 marginals making the margin of the axils. In Xenaster the first and 

 second pairs of proximal inframarginals are enclosed, while in Pro- 

 mopalseaster there are from one to three pairs in each interbrachial 

 area. As these areas enlarge, it follows that more and more of the 

 adambulacrals must also take part in the expansion. These ossicles 

 are therefore called interbrachial adambulacrals. Further enlarge- 

 ment, but not to any marked extent, takes place through the insertion 

 of a variable number of accessory disk interbrachials. This is best 

 seen in Trimeraster and Xenaster. 



Abnormal enlargement, but to a small extent, of the interbrachial 

 areas takes place through oral crowding entirely of adambulacrals in 

 pairs and through the increase in size of some of these ossicles. This 

 type of development is known only in Anorthaster. 



It has been shown how enlargement of the interbrachial areas comes 

 about through oral crowding of the axillaries, proximal inframargi- 

 nals and adambulacrals pieces of the primary skeleton but the 



