NORTH AMERICAN EARLY TERTIARY BRYOZOA. 441 



The ovicells have the aspect of the gonoccia of the Adeonidao. Employing 

 sections it may be seen (pi. 57, fig. 10) that this aspect is deceptive and that the 

 ovicells are hyperstomial and open in the peristomie; only the form of the peri- 

 stomice is changed. This change is in certain relationship with the physiologic 

 peculiarity which is difficult of analysis and of precise description. We believe 

 that the ovicelled zooecia exist only for the larva. After the passage of the eggs, 

 the polypide and the. compensatrix disappear by histolysis, so that the peristomice 

 lunar-crescent in shape, serves only for the escape of the larvae. It is, in fact. 

 very difficult to suppose the persistence of a polypide, in the absence of hydrostatic 

 apparatus (rimule) and in a peristomial system so long and so complicated. More- 

 over, these same zooecia are provided with a much reduced avicularium ; this organ 

 was useful in nutrition ; its reduction proves its inutility and consequently the dis- 

 appearance of the polypide. There is nothing analogous in the recent species. 



The species of this genus are quite polymorphic. Their classification has re- 

 quired much time, much patience, and much trouble. The alterations due to fos- 

 silization often occasion problems difficult of solution. 



The individuals were very voracious; they therefore did not persist in the 

 successive stages of a region. 



These ovicells are not without analogy with those of the Tubucellariidae 

 (text fig. 159 E) and the histological phenomena ought to be somewhat analogous. 

 The Tubucellariidae are always provided with an ascopore and not with a spiramen, 

 that is to say, a pore which opens into the compensatrix itself and not into a 

 peristomie. 



METRADOLIUM LABRATULUM, new species. 

 Plate 55, figs. 8-11. 



Description. The zoarium is free, bilamellar. with rounded and bifurcated 

 fronds. The zooecia are little distinct, elongated, elliptical; the frontal is a 

 tremocyst with tubules placed on a very thin olocyst in which very thin perfora- 

 tions correspond to the tubules. The apertura (interior) is orbicular; the peri- 

 stomice bears a rimule-spiramen on the zooecia with an oral avicularium; the 

 peristomice is orbicular on the zooecia provided with an enormous proximal mucro 

 at the base of which is the spiramen. The avicularium is very much projecting, 

 oval, furnished with a pivot ; its beak is directed toward the exterior. 



Measurements. Peristomice ?/>e=0.16-0 20 mm. Zooecia j j ~Q g _ 



Variations. This species is curious and instructive. On the same zoarium 

 we may note the existence of a rimule and of a spiramen. both of which must 

 have the same function, namely, of conducting the water below the apertura to 

 sway the operculum and to open the compensatrix. One must not confound this 

 spiramen with the ascopore of the Microporellae ; the latter is the same opening 

 as that of the compensatrix. 



Here the large oral mucro replaces exactly the avicularium; it therefore has 

 the same nutritive use. This is why in many mucronated species we see the mucro 



