EEPOET ON THE THNICATA. 
157 
large size (PI. XXL fig. 5) and are nsnally ratlier opaque. In some sections of the 
genital mass their germinal vesicles and germinal spots are remarkably well seen, and 
a distinct dot inside the germinal spot is visible. Transverse sections through the post- 
abdomen behind the genital mass show that it is traversed by the usual double-walled 
thin septum, which is curved so that its lumen is sigmoid (PL XXL fig. 18). The 
remainder of the post-abdomen is almost entirely formed of opaque mesoderm cells. 
Figure 18 represents an oblique section, and the three cavities at its upper end are spaces 
which were occupied by the spermatic vesicles forming the lower part of the genital mass. 
Tylobranchion^ n. gerc 
Colony large and massive. 
Systems inconspicuous. 
Ascidiozooids large, elongated antero-posteriorly, and distinctly divided into 
regions. 
Test gelatinous. 
Branchial Sac large and well developed. Transverse vessels provided with 
numerous large papdlse. 
Dorscd Lamina represented by a series of languets. 
Alimentary Canal large. Stomach-wall folded longitudinally. 
Reproductive Organs forming a long post-abdomen. 
This genus is distinguished from the other Polyclinidse by the structure of the 
branchial sac. This organ is large and well developed, and it possesses numerous 
stigmata which vary considerably in size, but the most important point is that the 
transverse vessels, which are numerous and of large size, bear on their inner edges well- 
marked papillae projecting into the cavity of the sac. These papillae probably represent 
the connecting ducts of the Simple Ascidians, and if we regard the longitudinal vessels in 
the branchial sac of Pharyngodictyon mirabile as being internal longitudinal bars, then 
the present genus will occupy an intermediate position between that form and the other 
Polyclinidae as regards the development of this part of the branchial sac. Further 
remarks will be found under the description of the single species. 
Tylobranchion speciosum, n. sp. (PL XXII. figs. 1-17). 
The Colony is irregularly wedge-shaped, with the larger end uppermost. The base of 
attachment at the posterior end is comparatively small, and from this point the colony 
^ From. rvXog and 
