THE ARENIG AND LLANDEILO ROCKS OF ST. DAVID'S. 669 



about sixteen in the space of an inch, equal in width to one 

 third the interspaces ; anastomosing at frequent but somewhat 

 irregular intervals ; the connecting filaments slender, expanding 

 slightly at their junction with the branches, and forming 

 irregularly-shaped fenestrules about twice as long as wide ; 

 surface of branches longitudinally striated. 



This species seems somewhat nearly allied to the Quebec form 

 Dictyonema (Dictyograptus) irregularis, Hall, but differs from it in 

 its completely circular mode of growth, in its branches being much 

 further apart, and in their being more slender in proportion to the 

 width of the interspaces. Although only small fragments of D. 

 irregularis have been found, from the specimens figured by Professor 

 Hall, it could scarcely, in its entire form, have grown in the 

 radiating manner of D. Homfrayi, the Quebec species having its 

 branches conjoined at their proximal end into a wide main stem, 

 while in our Eamsey-Island form the branches are scarcely thicker 

 here than at any part of their length, except at their growing 

 extremities. 



This species is named after its discoverer, David Homfray, Esq., 

 of Portmadoc. 



Log. Lower Arenig, Eoad Uchaf, Eamsey Island. 



Dictyograptus, sp. PI. XXXVII. fig. 4. 



In the Lower Llandeilo rocks of St. David's fragments of a Dictyo- 

 graptus have been found, but they are too imperfect to enable us to 

 determine the species. It has, however, been thought desirable to 

 mention their occurrence, and to figure a specimen, as the genus has 

 not before been recorded as present in the Llandeilo formation. 



Loc. Lower Llandeilo, Abereiddy Bay (higher zone). 



EXPLANATION OF THE PLATES. 



The initials H. H., D. H., J. W. K, and J. H, refer respectively to Mr. 

 Henry Hicks, Mr. David Homfray, Mr. J. W. Kershaw, and Mr. John 

 Hopkinson, from whose collections the species here described are illustrated. 



Plate XXXIII. 



Illustrative of the Arenig Ehabdophora described by Mr. Hop/cinson (with the 

 exception 0/ Didymograptus Mcholsoni, described by Mr. Lapworth.) 



Figs. 1 a -1 d. Didymograptus extensus, Hall, sp. 1 a, natural size* ; lb,\c,ld, 



portions of the same, magnified 5 diameters. From Eoad Uchaf, 



Ramsey Island : Lower Arenig (coll. J. H.). 

 2 a-2 d. Didymograptus sparsus, Hopk. 2 a, natural size ; 2 b, 2 c, portions 



of the same, magn. 5; 2d, the distal end of a branch, nat. size. From 



Eoad Uchaf: Lower Arenig (coll. J. H.). 

 3a-3e. Didymograptus pennatulus, Hall, sp. 3a, nat. size; 3 b, 3c, portions 



of the same, magn. 5 ; 3 d, a very young individual, nat. size ; 3 e, part 



of a branch of a mature individual, nat. size*. From Eoad Uchaf : 



Lower Arenig (coll. J. H.). 

 4«-4 e. Didymograptus patulus, Hall, sp. 4 a, 4 b, nat. size. From Porth 



Hayog, Eamsey Island : Upper Arenig (coll. H. H.). 4 c, a branch, 



nat. size * ; 4 d, 4 e, portions of the same, magn. 5. From Llanvirn 



Quarry : Upper Arenig (coll. J. H.). 



* In this figure the artist has made the number of thecae too small. 



