84 BRITISH TUXICATA. 



numerous, with the terminal twigs red in the vicinity 

 of the orifices, and frequently red vessels are scattered 

 over various parts of the test. In some specimens 

 such coloured vessels are numerous everywhere, par- 

 ticularly in young individuals. The twigs at the 

 external surface are comparatively short, and are irregu- 

 larly enlarged and rounded at the extremities ; those 

 ramifying within the inner surface of the test are long 



t/ o 



and slender, usually swelling gradually towards the 

 ends and occasionally terminating in globular enlarge- 

 ments. 



The mantle (PL IV, fig. 2) is remarkable for the 

 purity and brilliancy of its colour, the rosy tint be- 

 coming almost crimson at the tubes, which though 

 short are distinctly developed, and are of about equal 

 length. The muscular fibres on the left side are fine, 

 numerous, and interwoven, the transverse ones pre- 

 dominating. 



The branchial sac (PI. XVII, fig. 2) extends the whole 

 length of the mantle, and consequently for some dis- 

 tance behind the visceral mass. It is minutely plicated, 

 the plicae being as numerous as the longitudinal bars. 

 The primary vessels are numerous, rather close to- 

 gether, pretty equal in size, with the exception of a 

 few large ones placed at widish intervals. The second- 

 ary vessels are shorthand not very regular, the stomata 

 varying a little in form, the ends being either pointed 

 or rounded. The longitudinal bars are stout; the 

 papillae are large and conical, rather crowded; those 

 at the intersection of the meshes are scarcely larger 

 than the intermediate ones ; the papillary membrane is 

 not much developed. The oral lamina is rather narrow 

 and of equal width throughout; it is closely ribbed 

 transversely, even to the bottom of the branchial sac, 

 the ribs being strongest on the right side, finely pectin- 

 ated from end to end, and having the tentacular points 

 on the left of the mouth forming a continuous series to 

 the bottom of the branchial sac ; they are closely set, 

 narrow, and pointed. The branchial tubercle (PL XX, 



