1895.] 



MU. r. A. BATHER ON TJINTACRIIOrS. 



983 



Here, moreover, it is to be noticed that the law stated above 

 applies equally to U. xvestfaUcus (tig. 3), and therefore probably is 

 the law for the whole geuus. 



It is unnecessary to describe the fixed secundibrachs in detail. 

 They are flat, thin" plates, with no trace of any axial canal, either 

 in section, or superficially in the form of an axial ridge. They are 

 about the same width as the immediately succeeding free brachials, 

 but are much higher. lu the latter respect, however, no line of 

 demarcation can be drawn between fixed and free, since the change, 

 though rapid, is not sudden. 



The fixed pinnules arising from the fixed secundibrachs unite 

 with one another and with the interdistichals (or interaxillary) 

 and interbrachial plates to form the interdistichal and interbrachial 

 (or interradial) areas of the dorsal cup. Each interbrachial area 

 of an adult contains 3 pinnules on either side ; and each inter- 

 distichal area contains 2 pinnules on either side (fig. 8). But only 



Kg. 8. 



rig. 9. 



Fig. 8.— Uintacrimts socialis, part of Brit. Mus. B 6627, ,/, showing fixed 

 clistichal3 and pinnules, interdistichals, and interpinnulars. 



Fig. 9.— Fixed pinnules of ditto from Brit. Mus. E 6527, q. T^"*^ 

 natural size. 



Both fiijures 



the proximal portions of the pinnules are fixed ; the distal portions 

 nnist have been freely movable and quite independent. The 

 amount of fixation, and the number of pinnules involved, increase 

 with age. A young individual, viz. q (fig. 9), shows only two pairs 

 of pinnules in an interbrachial area ; and the proximal of these 

 appears to have only 5 ossicles fixed or modified by fixation, 

 whereas an older individual, viz. d (fig. 10), clearly shows 9 in that 

 state. In old individuals, even a fourth pair of pinnules may have 

 been involved in the interbrachial area. The proximal fixed ossicles 

 of these pinnules are large flat plates, both wider and higher than 

 the succeeding free ossicles. They show no trace of any axial canal 

 in the proximal region. The flattened plates of the fixed portions 

 of the pinnules do not form horizontal rows across the areas, but 

 alternate with one another, assuming a hexagonal outline. 



