DICTYOPHYTON. 127 



recent Euplectella, and its Sponge character has been generally accepted by later 

 authors, including Prof. Hall, 1 who has placed it with several other genera in a 

 separate family of fossil reticulate Sponges ; but the structural features of these 

 genera are not stated with sufficient clearness for an opinion to be formed as to 

 their value, and the characters assigned to the family are equally indefinite. 



As a general rule, the examples of this genus only occur in the form of casts, 

 in which the structure of the Sponge-wall is represented either by depressed or 

 raised longitudinal and transverse lines, forming regular squares in an arenaceous 

 matrix, and a thin film extends over the areas between the lines. Some of the 

 raised lines are stronger than others, and mark out larger squares, within which 

 subordinate squares are indicated by lighter impressed lines. Both the longitudinal 

 and transverse lines are continuous, and the extreme regularity of their course 

 seems to indicate that their component spicules must have been firmly united 

 together. In these casts the wall of the Sponge appears to consist of but a single 

 layer of spicular tissue. 



Mr. R. P. Whitfield 2 has stated that the longitudinal and transverse fibres in 

 Dictyophyton cylindricum are composed of bundles of cylindrical spicules of various 

 sizes and of great length, but no mention is made of the presence of cruciform 

 spicules. Prof. Hall 3 also states that " the structure of the frond which characterises 

 every member of this family may be described as a reticulation of tubular spicules 

 forming rectangular meshes ;" but here again nothing is said of the form of the 

 spicules. This author also refers to three spicular layers in the wall — a middle layer, 

 which is uniformly reticulate, and inner and superficial layers, which show an oblique 

 and sometimes a radiate arrangement of spicules ; but these are not shown in any 

 example of Dictyojphyton which he has figured. 



Assuming that the lines shown in the casts of Dictyophyton are really composed of 

 fascicles of spicules, this genus is distinguished from Protospongia, Salter, by the fact 

 that the quadrate areas of the wall in this latter form consist only of individual 

 spicules. It differs likewise from Plectoderma, Hinde, in the regular arrangement 

 of the surface squares and in having the transverse lines of the wall of spicular 

 fascicles, instead of individual forms. 



The earliest appearance of Dictyophyton is in the Upper-Ludlow strata of this 

 country, it also occurs in the Middle Devonian of the Eifel, Germany, and New 

 York, and attains its greatest development in the Upper Devonian (Chemung 

 Group) of North America, and it is also present in the same formation (Psammites 

 du Oondroz) in the Ardennes (Barrois). It makes its last appearance in the Lower 

 Carboniferous of Ohio and Indiana. 



1 ' Thirty-fifth Annual Report New York State Museum ' (1884), p. 465. 



2 ' Bulletin No. 1, Amer. Mus. Nat. Hist.,' December, 1881, p. 19. 



3 ' Thirty-fifth Annual Report on the New York State Museum,' p. 465. 



