BRITISH PALAEOGENE BRYOZOA. 265 



25 species, belonging to 17 genera. If we take the case of" the species of a single 

 genus, we find the same point very instructively shown. Thus the genus Idmonea is 

 represented by only five specimens, of which two from Mr. Vine's collection appear to 

 me to be indeterminable ; Idmonea may be conveniently divided into three groups or 

 subgenera, and one of the three recognizable specimens belongs to each of these three 

 groups. This consideration ought to stimulate the search for more material, as the 

 specimens already known appear to represent but a fragment of the fauna. 



The high proportion of peculiar species in this fauna Avould not excite surprise in 

 any of the higher groups, except the Bryozoa ; but when we consider the vast range 

 both in space and time claimed for some species, a few words of explanation are 

 required. In the first place, rare though the Bryozoa are in the English beds, they 

 appear to have been even scarcer in contemporary deposits of other parts of the same 

 basin; the meagre lists given by Stremme (No. i), Marsson (No. 2), Michelin (No. i), 

 Milne-Edwards (No. 2), and Mourlon (No. i) show the paucity of Bryozoa at this time 

 in Northern France, Germany, and Belgium. 



The great range in time usually accorded to species of Bryozoa raises the general 

 question as to the value of species in this group ; their growth in colonies is the main 

 reason for the " lumping " tendencies of zoophytologists. In the Cheilostomata species 

 are usually founded, if only on one specimen, yet on hundreds of zocecia : in a colony 

 of this size great variation is inevitable ; many of the polypites are crushed out by 

 growth-pressure, and their zooecia are malformed or aborted ; the older zocecia become 

 immersed and lose their characters ; the younger zooecia at the tips of the branches 

 are immature. Hence it is easy to pick out two zocecia in a zoarium which diff'er far 

 more markedly than do two zooecia taken from different species ; but that no more 

 proves that the two species should be merged than that two species of frogs are 

 identical because they resemble one another more closely than they do the tadpoles 

 from which they have developed. 



Dr. Waagen — ' Pal. Indica' (xiii.), ' Salt Range Fossils,' iv. pt. 2, 'Geol. Eesults,' 1891, 

 pp. 235, 236 — has recently pointed out the disastrous effects that have been Avrought 

 by palaeontologists " lumping " species and neglecting slight but definite differences ; 

 and one worker on Bryozoa has recently expressed his doubts as to the accuracy of the 

 identification of recent and Cretaceous species. With this opinion I feel strongly 

 disposed to concur, but will here only say that, so far, I have seen no Cretaceous 

 species of Cheilostomata identical with a living one. If there are such constant 

 differences, it seems certainly advisable to recognize them by name, whether we call 

 them species, forms (Smitt), or mutations (Waagen). Unless this be done, if we 

 accept species as ranging from the Jurassic to the present, then we must abandon all 

 hope of deriving from the Bryozoa any assistance in the study of the geographical 

 distribution of the past, though the group presents characters that should give its 

 evidence great value. 



