CYCLOSTOMATA. 6ig 



the cell-mouths, and may be developed on the non-celluliferous 

 dorsal surface. The earliest undoubted species of Horner a occur 

 in the Cretaceous rocks, and there are numerous Tertiary and recent 

 types. 



The family of the Lichenoporidce comprises discoid polyzoaria, 

 which sometimes in process 

 of growth become confluent, 

 or may even become mas- 

 sive, the colony being fixed 

 by a broad adherent base 

 or by a narrow peduncle. 

 The zooecia are tubular and 



erect, and are in Complete Fig- 462.— Botrylloporasocialis, from the Devonian 



, , rocks of Canada. «, A small colony attached to a 



COntaCt throughout, Or are coral, of the natural size ; b, A single disc of the same, 



free towards their extrem- SSGL? fodgfLo rtion of the same > enlar § ed 

 ities. The walls of the 



zooecia may be porous or imperforate, and the proper zooecia may 

 be separated by intermediate cancellated or porous spaces. 



The most ancient type of the Lichenoporidce appears to be the genus 

 Botryllopo7-a of the Devonian rocks (Hamilton formation) of Canada and 

 the United States. In this genus (fig. 462) the polyzoary consists of 

 separate or confluent discs, which are attached by the whole of the under 

 surface to foreign bodies. Each disc shows a series of elevated ridges 

 or ribs, which radiate from a central area and are separated by inter- 

 vening furrows. The ribs carry the apertures of the tubular zocecia, and 

 the spaces between them appear, in well-preserved specimens, to be 

 minutely porous. The genus Lichenopora (Discopore/ia) comprises 

 forms in which the polyzoary has the form of a simple disc or of 

 numerous confluent discs, each disc being composed of tubular cells, 

 which are arranged in lines radiating from a central space, and separated 

 by intervening intervals. The zocecia are free towards their mouths, and 

 project above the general surface ; and the spaces between them, as well 

 as the central area, are occupied by smaller interstitial tubes or " cancelli." 

 In some forms, both the proper zocecia and the " cancelli" are provided 

 with delicate capitate " rays." In many structural features Lichenopora 

 closely approaches Heteropora, but the zocecia of the latter are in contact 

 throughout, and are not free towards their extremities. The forms with 

 confluent discs are sometimes separated to constitute the separate genus 

 Radiopora. The earliest types of Lichenopora (if considered as embrac- 

 ing Radiopora) appear in the Trias ; while the genus has Jurassic, Cre- 

 taceous, and Tertiary representatives, and still survives at the present 

 day. In the genus Domopora {Defrancid) the polyzoary is discoidal or 

 massive, simple or lobed, attached by the whole base or by a stalk, or 

 sometimes free. The zocecia are " disposed in radiating lines, consisting 

 of one or more series, on the free extremity of the stem or lobes " 

 (Hincks), and their mouths do not project above the general surface. 

 The walls of the zocecia are penetrated by numerous delicate tubuli. 

 This genus should perhaps be removed to the family of the Heteroporidoe. 

 The species of Domopora appear first in the Jurassic, and are abundant 

 in the Chalk, while Tertiary and living forms are also known. 



