202 



CCELENTERATA. 



partitions or "tabulae" (fig. S3, D). Very commonly the tubes exhibit 

 more or fewer inwardly directed partitions, which extend to a greater or 

 less distance into the cavity of the tube, and are the result of the cleavage 

 or " fission " of the tubes. The species of Solenopora are entirely con- 

 fined to the Ordovician rocks, and sometimes occur in such profusion as 

 to give rise to beds of limestone. The best known species (viz., 5. 

 cojnpacta) has been recognised in the Ordovician limestones of Russia, 

 Britain, the United States, and Canada. 



Both Mitcheldeania and Solenopora are remarkable for the extraordi- 

 nary minuteness of their component tubes, and in this respect they differ 

 from all known Ccelenterates. Apart from this feature, they present in 

 their structure a general resemblance to forms like Parkeria, and they 



Fig. 84. — Sertularida. A, Portion of the colony ot Difihasia (Sertularia) tamarisca, of the 

 natural size, showing hydrothecee and female ovarian capsules (gonangia). b and c, Portions of 

 different branches of the same, enlarged : k, Hydrothecse ; a, Male gonangium; g, Female gon- 

 angium. (After Hincks.) 



may therefore be provisionally placed among the Hydroid Zoophytes. 

 In some points, however, the genus Mitcheldeania would appear to be 

 rather related to the Hydrocorallines. 



Among the other forms which have been referred to the Corynida are 

 the genera Corynoides and Palozocoryne. The former of these may, per- 

 haps, be best regarded as an abnormal type of the Graptolitoidea, and 

 will be alluded to in speaking of these organisms. The genus Palceo- 

 coryne, on the other hand, is founded upon structures which belong to the 

 fronds of certain of the Fenestellidce, and it will therefore be treated of 

 in dealing with the Polyzoa. 



