ESCHARIDjE. 73 



Genus 4. RETEPOKA, Imperato. 



Polyzoario foliaceo, reticulato, infundibuliformi seu contorto, subpedunculato. Cellulis 

 decumbentibus superficie superior! seu interim tantura se ostendentibus. 



Polyzoariurn. foliaceous, reticulate, infundibuliform, or contorted, subpedunculate ; 

 cells decumbent, opening on the upper surface only. 



RETEPORA, Imperato; M, Edwards (note in Lamarck, An. S. Vert., 2d ed., ii, p. 2/5) ; 



Johnston; Busk; D'Orbiyny ; Risso ; Reuss. 



(pars), Lamarck ; Lamouroux ; Blainville ; Hagenow ; Goldfuss. 

 MILLEPORA (pars), Linn. ; Pallas, &c. 



The peculiar conformation of the polyzoary in Retepora, renders the species of which 

 it is composed very readily recognisable generically. It is far otherwise, however, with 

 respect to the diagnosis of the species themselves. Even in recent and living forms, and 

 under the most favorable conditions of age and growth, it is not always very easy to 

 obtain a satisfactory view of the minute characters upon which alone the distinctions 

 between different species certainly rest. In the case of fossil specimens this difficulty 

 is of course greatly increased, owing to the usually fragmentary condition in which 

 they occur, and to the circumstance that the minute characters are either worn off or 

 concealed by encrusting matter, so as to be rendered indistinct. The characters, there- 

 fore, here assigned to what would appear to be distinct species, must be understood as 

 liable to future correction. The careful consideration of numerous specimens has shown 

 pretty clearly that the forms described are distinct, inter se, but whether better absolute 

 characters for each may not hereafter be assigned, cannot be affirmed. 



The only genus with which that of Retepora can be confounded is Hornera, several 

 species of which affect a very similar habit of growth, and consequently were formerly 

 regarded as congenerous with the true Retepores. The distinction between them appears 

 to have been first indicated by M. M. Edwards (1. c.). One broad distinction, obvious to 

 the naked eye, exists in the circumstance that in the true Retepores the polyzoary is com- 

 posed of pretty nearly equal-sized branches, anastomosing so as to constitute elliptical 

 meshes, whilst in the reticulate Ilornerce, it consists of tolerably straight, subparallel, 

 wider branches, connected at uniform distances by short transverse ramuscules, at right 

 angles to the former, by which arrangement the meshes or fenestras are rendered more 

 or less oblong or triangular. 



In Retepora the cells open on one side only of the polyzoarimn ; the opposite or dorsal 

 surface being smooth, or sometimes very finely grannlous, and divided, as it were, into 

 sections by lines, (vibices] which are usually raised above the surface. These lines do not 



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