VIl] PACHYTHECA. 203 



for a brief notice. Pachytheca is too doubtful a genus to 

 justify a detailed treatment in the present work. Although, as 

 I have elsewhere suggested \ we are hardly in a position to 

 speak with any degree of certainty as to its affinity, it is not 

 improbable that it may eventually be shown to be an alga. 



Without attempting a full diagnosis of the genus, we may 

 briefly refer to its most striking characters. 



Pachytheca usually occurs in the form of small spherical 

 bodies, about '5 cm. in diameter, in Old Red Sandstone or 

 Silurian rocks. In section a single sphere is found to consist 

 of two well marked regions; in the centre, of a number of 

 ramifying and irregularly placed narrow tubes, and in the 

 peripheral or cortical region, of numerous regular and radially 

 disposed simple or forked septate tubes. The tubular elements 

 of the two regions are in organic connection. 



The name was proposed by Sir Joseph Hooker for some 

 specimens found by Dr Strickland'' in the Ludlow bone-bed 

 (Sihirian) of Woolhope and May-Hill. Examples were sub- 

 sequently recorded from the Wenlock limestone of Malvern and 

 from Silurian and Old Red Sandstone rocks of other districts. 

 Hicks'' found Pachytheca in the Pen-y-Glog grits of Corwen in 

 association with Nematophycus, and the two fossils have been 

 found together elsewhere. This association led to the sug- 

 gestion that Pachytheca might be the sporangium of Nemato- 

 phycus, and Dawson^, in conformity with his belief in the 

 coniferous character of the latter plant, referred to Pachytheca 

 as a true seed. 



The best sections of this fossil have been prepared with 

 remarkable skill by Mr Storrie of Cardiff; they were carefully 

 examined and described by Barber in two memoirs' published 

 in the Annals of Botany, the account being illustrated by several 

 well executed drawings and microphotographs. 



Among other difficulties to contend against in the inter- 

 pretation of Pachytheca there is that of mineralisation. The 

 preservation is such as to render the discrimination of original 

 ijtructure as distinct from structural features of secondary origin, 



1 Seward (1)53). a Strickland and Hooker (58). » Hicks (81) p. 484. 



* DawBon (82) p. 104. » Barber (89) and (90). 



