THE AKENIG AND XLANDEILO ROCKS OF ST. DAVID'S. 639 



approximately on the same horizon on which they occur at Skiddaw 

 and Point Levis. 



A single genus belonging to each family proves the presence of 

 the Nemagraptidse, the Dicranograptidse, and the Phyllograptidse. 

 Both genera of the family Diplograptidse are pretty fully represented, 

 both coming in together in the Middle Arenig, as in Norway, Cum- 

 berland, and Canada. 



In spite of the great additions to our knowledge of the Graptolite 

 fauna of these beds made by these researches, a comparison of the 

 species given in the Table with those yielded by the corresponding 

 deposits of Skiddaw, Canada, Norway, and South Scotland, while it 

 shows many new forms, shows also a remarkable absence of some of 

 the most characteristic forms common to these widely separated 

 areas. As many of them occur, however, at Shelve and elsewhere, it 

 is more than probable that they will ultimately be detected in the 

 St. -David's area. 



The many remaining points of resemblance and contrast that may 

 be instituted between the fauna of these beds of St. David's and 

 their equivalents elsewhere, cannot here be even alluded to. The 

 most superficial comparison of the list of species given in the Table, 

 with those of the localities already so frequently adverted to, will at 

 once convince the palaeontologist that the general facies of the Grap- 

 tolitic fauna of any subformation of the area under consideration 

 is almost identical with that of the synchronous rock-groups else- 

 where. Perhaps the most patent result of these researches is the 

 circumstance that they clearly demonstrate that the Hydroida of 

 these ancient rocks, so long shunned or misinterpreted by the sys- 

 tematise are rapidly emerging from the obscurity which has enveloped 

 them, and will perhaps soon stand side by side with the better-un- 

 derstood Brachiopoda and Crustacea, as unerring exponents of the 

 true geological age of the most widely separated rocks in which they 

 are found. 



Terminology, &c. 



The discovery of the nature and function of the sicula, and of its 

 relations to the adult Graptoloid, has throivn great light upon several 

 disputed points in the morphology of the Bhabdophora ; and several 

 structural features, formerly believed to be of no special importance, 

 have been brought into great prominence by recent researches amongst 

 the Graptolites in general. This necessitates the employment of a 

 few new terms, and more precise definitions of some others that 

 have already obtained general acceptance. Only those, however, 

 will here be noticed which are immediately connected with the 

 forms described in the present communication. 



1. Sicula. — The chitinous covering of the free zooicl, " germ," or 

 embryo, in the Graptoloidea. Its original form was probably that 

 of a minute hollow cone ; but, in its present state, flattened upon 

 the stone, it usually presents the appearance of an elongate, trian- 

 gular film of chitine, in which a slender longitudinal thread is im- 

 mersed, one end of which is slightly projecting, 



Q. J. G. S. No. 124. 2 u 



