342 CHARACTERS AND DIVISIONS OF ALCYONARIA. 



of large size ; and consists of long, tubular, prismatic corallites, 

 which are not only in close contact throughout, but are so com- 

 pletely incorporated by fusion of their walls that rough fractures 

 almost always expose the interior of the tubes. The corallites are 

 all of one kind, are irregularly polygonal, and possess completely 

 imperforate walls. The tubes are traversed by a few complete 

 tabulae (fig. 222, b), which are commonly developed periodically at 

 successive levels ; but there are no septa. The increase of the cor- 

 allum is by fission of the old tubes ; and hence in cross-sections 

 (fig. 222, a) it is common to observe imperfect longitudinal partitions 

 projecting into the visceral chambers of the corallites, and indicating 

 the uncompleted fission of the tubes. 



The zoological affinities of the genus Chcetetes are quite uncertain, 

 and it is not even clear that it is referable to the Alcyonaria. 

 Many authorities regard the genus as belonging to the Polyzoa 

 rather than to the Coelenterata ; but the nssiparous mode of its 

 development, and the total absence of pores or canals connecting 

 adjoining tubes, would militate strongly against its being referred to 

 the former group. 



Auloporim:. 



This small and imperfectly understood group comprises the 

 Palaeozoic genera Aulopora, Cladochonus (Pyrgia), and Monilipora, 

 of which the first may be taken as the type. The corallum in 

 Aulopora (fig. 223, a) has the form of a creeping, branched, or 

 reticulate system of tubes, attached by the whole of the lower sur- 

 face to the exterior of a shell, coral, or other foreign body. The 

 prostrate stems or stolons send up at longer or snorter intervals 

 reclined tubular or trumpet-shaped corallites, which are free termin- 

 ally, and do not grow up to form a fasciculate mass. As regards 

 their internal structure, the corallites are commonly furnished with 

 curved or horizontal tabulae, and rudimentary septa are present in 

 the form of marginal striae or rows of tubercles. The walls of the 

 corallites are imperforate, and the mode of increase of the corallum 

 is by stolonal gemmation. The species of Aulopora range from 

 the Ordovician to the Carboniferous inclusive. 



Cladochonus (Pyrgia) closely resembles Aulopora, but the colony 

 is only attached to foreign bodies by isolated points of attachment, 

 and grows in an erect manner rather than as a creeping network. 

 In its young state, the corallum consists of a single trumpet-shaped 

 corallite (fig. 223, c and d), attached by processes springing from 

 its lower surface. Monilopora possesses an Auloporoid corallum, 

 which is attached parasitically to the stems of Crinoids in a ring-like 

 manner, the walls of the trumpet-shaped corallites having a peculiar 



m^ 



