Tlte Barrettia Beds of Jamaica. 509 



Tindulating in form, following the irregular surface of the lower 

 valve. It is generally only in young individuals, especially of the 

 species B. monilifera, that the decoration of the surface of the top 

 valve can be seen. In older specimens the decoration both of the 

 lid and of the sides of the lower valve is 'generally obliterated by 

 erosion or overgrown by oysters and other adherent organisms. 

 The top valve, in fact, is so thin that in the niajority of specimens it 

 has been partly or com]jletely crushed into the visceral cavity during 

 the process of fossilization. 



The question arises in one's mind why did Barrettia develop the 

 exaggerated system of infoldings of the outer layers of the shell, 

 the moniliform rays or pallial duplicatures, as they have been termed, 

 which forms the most conspicuous feature of the genus, and which 

 in polished cross-section resemble a string of beads. The answer 

 seems to be that the animal was one of a relatively small size ; the 

 diameter of the living chamber in my specimens is generally about 

 one-third the diameter of the top part of the lower valve. In fact, 

 several of these large shelly structures seem to have been built up 

 by an animal not much larger than a good-sized modern oyster. 

 The hinge teeth of the upper valve are consequently comparatively 

 small and weak, and the lid would require some additional method 

 of fixation to the lower valve when closed. This was accomplished 

 by numerous projections from the lower surface of the lid fitting 

 into cavities on the surface of the lower valve round the edges of the 

 living chamber. These cavities or depressions occiipy the spaces 

 between the radial infoldings and a system of vertical shelly walls 

 arranged concentrically and more or less parallel with the outer 

 wall. A large number of projections serving the pur]-)ose of hinge 

 teeth are therefore develoi:>ed on the under surface of the lid, and fit 

 closely into the cavities below, whose walls are formed by the rays, 

 as I have already described. 



The reason which impelled Barrettia to build up such an 

 enormously greater amount of shell in comparison with the size 

 of the animal than does Hippurites, is not so easy to see, but 

 presumably it had to withstand greater battering from waves, or 

 to adopt protection against some special enemy. 



Whitfield describes the moniliform rays, or lines of beads as they 

 were called by Woodward, as vertically sejjtate tubes. ^ He also 

 describes the first pillar as a longitudinal septate tube extending 

 the entire length of the body. The second pillar he describes as a 

 fosset-like body, and compares it with the fosset of a palaeozoic 

 cup-coral. In fact, he can find no evidence of a molluscan nature 

 except the presence of the upper valve in Barrett's original specimen. 

 He also attempts to describe the visceral cavity, but a glance at his 

 specimen figured on pi. xxxv shows an example with the living 

 chamber badly weathered. If one obtains specimens which allow 



^ " Observations on the genus Barrettia," loc. cit., p. 239. 



