BRITISH PAL^OGENE BETOZOA. 235 



of the genus, though it has been overlooked or merely mentioned by subsequent authors. 

 Unfortunately, however, the name was preoccupied among Coleoptera by Kirby in 1837, 

 and as it is still in use for that group the Bryozoan genus must be renamed. The 

 nature of the zooecia of the lower surface is somewhat puzzling ; four explanations 

 of their nature may be offered : — First : the zoarium may be fixed, probably in mud ; 

 in that case the peripheral zooecia would be normal ; but as they became more central 

 by the growth of the colony they would gradually become aborted and their apertures 

 closed ; the distribution of the under zooecia supports this view. Second : the zoarium 

 may be free and the modified zooecia of the lower surface may all be swimming vibracula 

 instead of normal zooecia ; in that case the thickening of the peristome would be due 

 to the necessity for greater muscular attachments. Third : the zoarium may be fixed 

 by radical fibres or tubes given ofi" from the modified zooecia. And fourth : the zoarium 

 may be free and the peculiar lower zooecia may be gonoecia, as the thickened and con- 

 tracted apertures resemble those of elements in other genera, such as Teichopora, which 

 appear to be clearly gonoecia. So long as the genus remains known only by extinct 

 species it may be impossible to decide between these views, but I am inclined to accept 

 the first, though there are points that make for the second. 



The geuus differs from the rest of the group by its bilaminate nature and the structure 

 of the inferior zooecia. It is possible that it ought to be subdivided, one branch including 

 the type species and all the rest of those in which there is a vibracularium to every 

 zooecium. 



Species 1. Biselen.uiia offa ^ n. sp. 



Diagnosis. Zoarium: a small circular disk, thickest in the middle and tapering 

 towards the periphery. 



Zooecia irregular in form and distribution ; a group of small ones occurs in the centre ; 

 the largest are in a circle at a little distance from the margin. The opesia are large 

 and elliptical, surrounded by a thickened margin ; some of the opesia are slightly trigonal. 

 The vibracularia are very irregular in distribution ; they resemble the normal zooecia 

 in general foi'm, but the rim is thicker in proportion to their size. 



The zooecia of the lower side vary from being identical with those of the upper side 

 to being quite closed ; all intermediate forms occur, but a spathulate form with the 

 aperture remaining as a slit or small pore is the commonest. Some of the vibracularia 

 have the very typical auriculate appearance. 



Distribution. Barton Beds, Barton. 



Ti/2)e. Brit. Mus. No. 49759. Edwards Coll. 



Figures. PI. XXX. fig. 4. Zoarium of type specimen : upper surface. Fig. 4«. Part 

 of another specimen : under surface. Fig. 5. Upper surface of another zoarium. 



' Offa, a bun. 



VOL. XIII. — PAKT VI. No. o. — June, 1 893. 2 m 



