224 Dr. J. W. Gregory — A Revision 



general facies of the Cjclostomatous faunas of the various 

 geological systems is, however, strikingly different; this can 

 at once be seen by a comparison of lists of the genera. If 

 the genera vary it is almost certain that the species must do 

 so likewise. The specific characters are variable and slight. 

 But if we examine good series of specimens, and compare the 

 normal types of the zooecia and equivalent zooecia in the two 

 zoaria, then certain fairly constant differences appear. Thus, 

 if we take a Jurassic specimen in which the zoarium contains, 

 say, two hundred zooecia, and compare it with one of a closely 

 allied recent species with as many zooecia, it is not improb- 

 able that one zooecium in each may be found to be identical. 

 But that does not seem sufficient reason for ignoring the 

 constant differences between the majority of the zooecia in 

 each. The embryos and young forms of different species of 

 ]\lollusca are often indistinguishable; but that does not lead 

 nialacologists to merge the species when there are definite 

 differences in the adults. The variations in the zooecia of a 

 zoarium of a bryozoon is an analogous case to this ; some 

 zooecia are young and immature, otliers are cramped and 

 malformed. To draw up a diagnosis which shall accurately 

 describe each zooecium in a colony, and shall at tiiesame time 

 be sufficiently definite to characterize the species, is impossible. 

 Nevertheless, if we lake tiie normal adult zooecia and compare 

 equivalent ones in different species, there seems sufficient 

 reason for supporting tiie practical validity of species in 

 this group. 



II. Revision of the Species. 



The genus Stomatopora affords a very convenient illustra- 

 tion of the difficulties, but yet of the possibilities, of the 

 diagnosis of the Cyclostomata. It is, moreover, the first 

 genus represented in the Jurassic that comes under considera- 

 tion in the preparation of a catalogue of the Jurassic Bryozoa. 

 It may be useful to publish a synopsis of each of the leading- 

 genera as they are finished. 



Family Tubuliporidse. 

 Genus Stomatopora, Broun, 1825. 



Aleeto, Lamouroux, 1821. 

 Aulo2)07-a, pars, Goldfuss, &c. 



Diagnosis. — Tubuliporidaj with the zooecia forming flat 

 adnate zoaria, composed of uniserial lines. These branch 



