TEEPOSTOMATA. 
125 
Monticuliporidae the statement that the zooecia have ‘‘ thin and 
prohahly minutely perforated walls, the peculiar granular structure 
exhibited in thin sections being strongly indicative of an originally 
porous condition.” The absence of radiating spines is not true 
of all Monticuliporoids, and their presence has only been demon- 
strated exceptionally among acknowledged Trepostomata. The 
spines are so delicate that they are easily destroyed and rarely 
preserved in fossils ; even in living species they sometimes escape 
detection, for their occurrence in the recent Heteropora from 
Xew Zealand was overlooked by Mr. Waters.^ The third character 
— the difference in structure between the larger and smaller 
individuals in dimorphic colonies — is equally true of the zooecia 
and mesopores of dimorphic Trepostomata. 
Ulrich, in 1882, included the Cerioporidee in the Trepostomata; 
hut in 1900“ he separated them on what seem to me inadequate 
grounds. He, however, then included in his Cerioporidae fossils 
such as Neuropora, which I do not regard as a Bryozoan hut as 
a Hydrozoan. The grounds for Ulrich’s decision are given in the 
following quotation : “ The Cerioporidae greatly resemble many 
of the Palaeozoic Trepostomata, hut, as a rule, may be readily 
distinguished by the complete amalgamation and porous nature 
of their zooecial walls.” 
In many Mesozoic Bryozoa here included in the Trepostomata, 
the walls are as distinct and no more porous than those of some 
typical Palaeozoic members of the group. Ulrich includes the 
qualification “as a rule,” thereby admitting that the grounds of 
his separation are not constant. 
The order Trepostomata appears to be, therefore, Mesozoic as^ 
well as Palaeozoic. It is well represented in the Jurassic, andi 
some survivors from that fauna and numerous fresh genera 
lived in the Cretaceous. The Trepostomata are scarcer in the 
Cainozoic, the group having been dwindling since the Palaeozoic, 
except that they also shared in the great development of tubular 
Bryozoa that happened in the Cretaceous era. 
The Trepostomata comprise many of d’Orhigny’s Crescisidae 
hut in that family he includes within the genus Heteropora many 
^ Xicholson, Monticulipora, 1881, p. 76. 
2 In Zittel-Eastman. “ Textbook of Palgeontology, ” 1900, vol. i. p. 266. 
