356 MISCELLANEOTJS SEEDS [CH. 



but which may be useful in distinguishing seeds which cannot be 

 satisfactorily separated by microscopic features. The superficial 

 cells have very thick walls and present an appearance similar to 

 that of the sclerous cells in the testas of some petrified specimens. 

 The larger seed shown in fig. 501, D, from Commentry, 

 described by Eenault and Zeiller^ as Cardiocarpus sclerotesta, is 

 another example of Cordaicarpus as now defined. 



IV. Miscellaneous Seeds. 



In this section are included genera based on characters of 

 comparatively little morphological importance; their claim to 

 treatment under a common heading is that we know little or 

 nothing of their anatomical features or of the parent-plants. 



i. Ribbed seeds. 



The seeds represented by the following genera possess ribs 

 or flanges usually in multiples of three; the transverse section 

 may be triangular, hexagonal, or polygonal. The symmetry is 

 generally radial, but it is difficult to determine whether a slight 

 departure from the radiospermic form is an original feature or the 

 result of pressure. Some of the ribbed seeds with which we are 

 now concerned are no doubt closely allied to Trigonocarpus and 

 should be included in the Trigonocarpales, but others may well 

 be distinct forms. 



Genera : Hexagonocarpus, Decagonocarpus, Polypterocarpus, 

 Rhynchogonium, Boroviczia, Diplopterotesta, Musocarpus, Holco- 

 spermum. 



Hexagonocarpus. Renault. 



This generic name is adopted by Eenault^ for casts from 

 Commentry which, while probably identical with Brongniart's 

 Hexapterospermum, afiord no confirmatory anatomical evidence. 

 The species Hexagonocarpus crassus^ is represented by casts or 

 nucules characterised by six well-defined ribs and grooves with 

 a length of nearly 3 cm. In this genus may be included the cast 

 from the Coal Measures of Lancashire described by Williamson* 



1 Renault and ZeiUer (88) A. PI. Lxxn. fig. 3. 



2 Ibid. p. 649. ' Ibid. PI. Lxxn. figs. 53—55. 

 ' WaUamson (77) B. p. 253, PI. xvi. figs. 115 a, 115 6. 



