236 SIDNEY V. IIARMER. 



criticism appears to me to lose sight of the fact that the 

 supposed ooecial cavity of Hincks and Busk was more than 

 the lateral recesses. I have no evidence which justifies me 

 in expressing a definite opinion on this subject, although I 

 have no reason to think that the dimorphism is connected 

 with sexual differences. The ratio between the A and b 

 forms is curiously different in different species. In the 

 majority of forms characterised by the possession of b opercula 

 with A-shaped thickening, the b form is found in large 

 numbers. In those with a fl-shaped thickening, on the 

 contrary, this form of zooecium is always rare, and appears 

 to be disappearing altogether. This leads to the cases (S. 

 neozelanica, etc.) in which no b zooecia can be found at all. 

 In S. alveolata the reverse condition obtains, the a form 

 being very rare, almost the entire colony being composed of 

 b zooecia. This is the only species in which I have been able 

 to obtain spirit material in which the polypides are present; 

 and it is obviously not well adapted for the examination of 

 the meaning of the dimorphism. I have at present been 

 unable to detect any trace of reproductive organs, but I am 

 able to state that polypides occur in the comparatively few 

 A zooecia, and I have not noticed any difference between 

 these and the polypides of the b form. 



Steganoporella possesses no avicularia, although these 

 are present in its ally Thalamoporella. I am at present 

 inclined to regard the b form of zooecium as representing an 

 avicularium, and probably that form of avicularium, common 

 n the extinct allies of Steganoporella, which Jullicn 

 1881) ttrms an "onychocellarium." I am disposed to do 

 this largely in consequence of the similarity between its 

 muscles and those figured by Jullien ('Cap. Horn,' pi. xii, 

 fig. 3) in the avicularium of Beania magellanica. I am 

 aware of the fact that the occurrence of avicularia containing 

 polypides would be very unusual. 



A tentative suggestion with regard to the function of avi- 

 cularia may be here made. Anyone who examines encrust- 

 ing and Escharan forms of Polyzoa must be struck by the 



