SIPHODICXTUM. 
363 
Type Species. 
Siphodictyum Lonsdale, 1849. Lower Greensand: Isle 
of AVight. 
Affinities. 
This genus is the oldest of the Horneridog, and is important, as it 
forms a link between that family and the Idmoniida?, while in one 
respect it approaches the Petaloporidce. It is a typical Hornerid, as 
it has the reverse face covered by a reticulate series of ridges, the 
apertures arranged in subregular transverse series, and the walls 
pitted by maculae. By the arrangement of the maculae it resembles 
Petalopora^ as they occur in regular single or double series below 
the apertures. I was therefore at first disposed to regard the genus 
as a Petalopoiid, with the apertures confined to one surface. 
Lonsdale gave a detailed and accurate description of the 
structure of the type species. 
The genus difters fi’om JPornera^ its nearest ally, by the 
Petaloporoid arrangement of the maculae. 
Siphodictyum gracile (Lonsdale), 1849. 
SyN0NY3IT. 
Cricopora gracilis, Fitton, 1847. Sect. Atherf. : Quart. Jouru. GeoL Soc. vol. iii. 
p. 302. 
-Siphodictyum gracile, Lonsdale, 1849. Foss. Zooph. Atherf. : Quart. Jouni- 
Geol. Soc. vol. V. p. 94, pi. v. figs. 16-23. 
,, ,, Morris, 1854. Cat. Brit. Foss., 2nd ed., p. 64. 
Siphodictgon ,, Bristow, 1889. Geol. I. Wight, 2nd ed., p. 262. 
,, ,, Xoruian, 1890. Geol. I. Wight, p. 38. 
Hornera^ ,, Vine, 1884. Fourth Eep. ; Eep. Brit. Assoc. 1883, 
p. 169. 
Di.\gxosis. 
Zoarium of thin, cylindrical, dichotomous branches, which do not 
anastomose. Most of the surface marked by longitudinal ribs. 
Apertures in transverse series or quite irregular. 
Distribution. 
Lower Greensand — Folkestone Beds (Ppper Aptian) ; Folkestone. 
Lower Gryphea Beds in Ferruginous Sands (Lower 
Aptian) : Whale’s Chine, Atherfield. 
