INTRODUCTION. 31 



(1887). This could not be done without reference to and 

 description of many Cretaceous forms. I hope, therefore, to 

 consider the question in greater detail when describing the Cre- 

 taceous Bryozoa. Classifications founded only on recent species 

 are of little value ; for the existing fauna includes but a small 

 number of Cyclostomata, which are not representative of the 

 varied types met with in the Mesozoic. 



The first serious attempt to classify the Cyclostomata was 

 that of D'Orbigny in 1851, whose "Bryozoaires Centrifugines " 

 include this order. He divided this group into two suborders, 

 one of which practically corresponds with Busk's division, the 

 Articulata. The rest, or the Centrifugines empates, consists of the 

 Cyclostomata, Inarticulata, and of many Paleozoic forms now 

 included in the Cryptostomata. His first division was named the 

 OpercuUs, but as it was based on an entire misconception of the 

 forms included within it, it may be ignored. The rest, however, of 

 D'Orbigny's major divisions seem to me to be based on truth. His 

 scheme is as follows : 



Division B. Fasciculines. Zooecia grouped in bundles. 



Fam. 1. Fascigeridce. No mesopores. 



2. Fasciporidce. JVXesopores present. 

 ,, C. Tubulines. Zocecia tubular and more or less free. 



Fam. 1. Tubigerida. Apertures in transverse series. 

 ,, 2. Sparsidce. Apertures scattered. 

 ,, 3. Clausidce. Aborted zocecia present. 

 ,, 4. Crisinidce. Zooecia on one face, and intermediate 



pores on the other. 

 ,, 5. Caveida. Zocecia and mesopores on one face, and 



intermediate pores on the other. 



,, D. Foramines. Zocecia not projecting above surface of zoarium. 

 Fam. 1. Ceidce. Zocecia funnel-shaped. 

 ,, 2. Cavidce. Zocecia not funnel-shaped. 

 ,, 3. Cytisida. Intermediate tubuli. 

 ,, 4. Crescisidce. Mesopores present. 



This classification seems to me to contain a good deal of truth. 

 It recognizes the importance of mesopores, and that the forms 

 included within it may be divided into three main types of 

 zoarial structure those in which the zocecia occur as masses of 

 tubes (Trepostomata or Kectangulata), in bundles, and in varied 

 congeries of tubes. 



