American Palceozoic Bryozoa. 



251 



opercula which have been left behind in the tubes at successive stages 

 of growth. 



In longitudinal sections (Plate XL, fig. Qa) the difference in structure 

 between the proper and interstitial tubes is conspicuous, both sets 

 being crossed by complete horizontal diaphragms, which are much 

 more numerous in the small tubes than in the large ones. Just above 

 the point of development, the young tube is crossed by closer set 

 diaphragms than in any portion of its length after it has attained the 

 mature size. This feature gives the young tubes the appearance of 

 interstitial tubes, and they may really have been of that nature in their 

 undeveloped stage. In the axial region the diaphragms in the tubes 

 of full size are distant from each other about one tube diameter, while 

 in the peripheral region the} 7 are about half that distance apart. 



Transverse sections (Plate XL, fig. 66) show that the tubes in the 

 axial region maj r be divided into two sets, one consisting of sub- 

 cylindrical tubes of nearly uniform size (the fully matured tubes), and 

 the other of smaller, unequal and angular tubes (the young tubes in 

 various stages of development). 



Nicholson (Pal. Tab. Cor. p. 301, 1879, and Genus Mont., p. 91, 

 1881) regards Callopora, Hall, as u unquestionably congeneric" with 

 Fistidipora, McCoy. After describing and figuring (Plate XX, figs. 

 6-66) the characters of V. elegantula, Hall, the type of the genus 

 Callopora, such an assertion scarcely merits a serious verbal refutation. 

 His idea of Callopora is clearl}" based upon Fistidipora incrassata, 

 which he originally referred to the former genus. But because the 

 Callopora incrassata proved on investigation to have the same general 

 structure as Fistidipora minor, McCo} 7 , it certainly does not follow 

 that Callopora is a synonym for McCo}''s older name Fistidipora. 

 In both the works cited he makes the rather equivocal declaration 

 that "the identity of Fistidipora, McCoy, and Callopora, Hall, has 

 long been more than suspected," and further adds, u having carefully 

 examined specimens of F. minor, McCo} r , the t} T pe of the genus 

 Fistidipora, and having compared these witli typical examples of 

 Hall's genus Callopora, from the Silurian and Devonian rocks of 

 North America, I am satisfied that the two are unquestionabh* con- 

 generic, and that both must be united under the older name of Fistuli- 

 pora, McCoy." To an inquisitive searcher after the truth, there is 

 nothing satisfactory in either of the quoted statements, and so far as 

 I am able to judge, nothing that he has said upon the subject, has 

 any direct bearing upon the actual point at issue, i. e., the suspected 



