140 INVERTEBRATE PALAEONTOLOGY 



thread-like colonies of the Ctenostomata (e.g. Vinelld) 

 are more abundant in the Lower Palaeozoic than at 

 later horizons, but they never attain prominence. Cyclo- 

 stomata seem to be represented in the Ordovician 

 and Silurian by the suborder Ceramoporoidea (e.g. 

 Ceramopora and Fistulipord] ; these somewhat problem- 

 atical forms constituted a large proportion of the 

 Polyzoan fauna of those periods. The abundant series 

 of forms known as Trepostomata are particularly char- 

 acteristic of the Lower Palaeozoic (though enduring to 

 the Permian). Under the name of " Monticuliporoids " 

 their massive colonies were long regarded as those 

 of Coelenterates of Alcyonarian affinities, but their 

 Polyzoan position seems established. Cryptostomata, 

 another exclusively Palaeozoic order, were less abundant 

 in the Lower than in the Upper periods, but Ptilodictya 

 sometimes occurs in profusion in the Ludlow Limestone. 

 This order was most probably ancestral to the Cheilo- 

 stomata, the predominant group of Tertiary and Recent 

 Polyzoa, which are not known from the Palaeozoic. 



(G) BRACHIOPODA 



In clearness and completeness the palaeontological 

 record of the Brachiopoda is superior to that available 

 for any other phylum. Not only do all members of 

 the group secrete shells suited for preservation, while 

 retaining some evidence of " soft " anatomy ; but there 

 is reason to believe that the earliest Brachiopoda known 

 are very nearly the simplest and most archaic of their 

 kind. In the Cambrian Rustellacea there is perpetuated 

 the primitive stage of evolution when adults were 

 encased in shells hardly different (save in size) from the 

 first post-larval " protegula " of later types. A large 

 proportion of known Brachiopoda are not only fossil but 



