part 3] 



THE KELESTOMIN.V.. 



205 



the underlying primitive characters which compose the common 

 ancestor. To do so presupposes a general knowledge of the various 

 subfamilies, and it is not proposed to enter into a description of 

 these, but to take the knowledge for granted, and to describe this 

 supposed ancestral form. The hypothetical ancestral Pelmatoporid 

 (fig. 1), as I conceive it, was a multiserial, incrnsting, unilaminar 

 cribrimorph Polyzoan. The cecia were dimorphic, that is, consisted 

 of normal oecia and avicularia ; the former of moderate size (about 

 *4 mm. long), and generally elliptical in shape. The extraterminal 

 front-wall was comparatively small. There was no secondary 

 interoecial tissue. The intraterminal front-wall consisted of about 



Fig. 1. — Diagram of a hypothetical Primitive Pelmatoporid. 

 X about 150 diameters. 



-Apertural spine. 

 Aperture. 



Apertural bar. 

 Avicularimn. 



Pelmatidium. 



Costa. 



Extraterminal front-wall. 



Termen. 



ten rather slender and very widely-separated costse with no lateral 

 costal fusions, but firmly united in a thin median line of fusion. 

 Close to this line of fusion were the upturned, hollow prolonga- 

 tions of the costse, which were of small diameter, so that their 

 broken ends form a row of pelmatidia on each side of the mid- 

 line. Each half of the apertural bar resembled the other costae. 

 The aperture was semicircular or super-semicircular in shape. 

 There were six small apertural spines. The avicularia were com- 

 paratively numerous, sporadically distributed in the interoecial 

 valleys, indifferently orientated, monomorphic, small and with 

 blunt, short apertures divided by a bar into a smaller, more 

 or less semicircular, proximal, and a larger, distal, or mandibular 

 portion. Such a form is diagrammatically represented in fig. 1, 

 and it is necessary to grasp its essential characters in order to 

 understand the following description of their modifications. 

 Q. J. G. S. No. 295. n 



