202 



valve and willi Iwo opesiula*, while the kenozoa>cia, spiingiiii; from the axial 

 zott'cia, have a small dejuesscd c'ryi)locy-sl perfoialcd i)y a pore. All septa have 

 a series of single-pored rosclte-plales. Hivalve (xrcia, in which each valve must 

 be considered a kenozoceciuni. No aviciilarid. 



Alysidium parasiticum Rusk. 

 Catalogue of Marine Polyzoa, Part I, Cheiloslomata, pag. 14, PI. XIV', 



figs. 6—9. 

 (PI. VII, figs. ;j a-3 o). 



The zooecia. which are rather elongated and trai)eziformly rounded, steadily 

 increase in hreadth towards the arched, distal margin. The aperture, the slightly 

 curved proximal margin of which is situated in the distal third of the zoa'cium, 

 is broader than long and has a glistening ridged distal margin, often with a 

 series of small tubercles. There is a membranous opercular valve and the <)|)er- 

 cular arch is situated in the free margin itself. Almost the distal half of the 

 frontal surface is furnished with a depressed cryptocyst, which also occupies the 

 region between the aperture and the distal margin of the zoreciuni. The post- 

 oral cryptocyst, which stretches more than half-way back between the aperture 

 and the proximal margin of the zooecium and which has generally a number 

 (most often 10 — 15) of glistening tubercles, is separated from the remaining arched 

 part of the frontal surface by a semi-elliptical boundary ridge, which is very low 

 in the middle but increasing in height distally and ending on each side at 

 one of the horn-like spines, from which it is separated by a small notch. The.se 

 two spines, situated at the margin of the zoa'cium oj)posite the aperture and 

 standing out almost vertically from the surface of the zoa-cium, have generally 

 a form resembling that of short cow's horns but are a little more compressed. 

 In their proximal inner i)art each of them has a small hole, apparently leading 

 into the inner cavity. On the proximal side of the aperture on each side is a 

 rather small, irregularly rounded opesiula, the inner margin of which nearly 

 always terminates in a short, most often rod-like jjrocess, seldom with two or 

 several points. The two opesiulae are always of dilTerent size, but while in the 

 axial zocecia this difference is slight, it is large in the others where the opesiula 

 facing the axis of the colony is twice the size of the other. Immediately on the 

 proximal side of the two opesiulie is an oblique, glistening stria, which is how- 

 ever rather indistinct in the axial zooecia, passing right across the zooecium to 

 the lateral margins. This stria, which in zooecia of the second and third order 

 is inclined towards the central line of the colony, originates from a low ridge 

 on the inner surface of the cryptocyst, and immediately distally to it is the limit 



