A new Crustacean from Stonesfield. 5 



Every systematic naturalist feels it incumbent on him to refer 

 the form he is describing to its proper position in the family and 

 class to which it belongs, but the palaeontologist knows well how 

 difficult it is, from a portion only of an animal structure — whether 

 vertebrate or invertebrate — to predicate with certainty its true 

 affinities. In the present case the genus Prosopon has been referred 

 to the Corystid(B, a group which nearly approaches to the Anomoura 

 of Milne-Edwards. Even upon the imperfect knowledge which the 

 present form gives us, we are led to perceive its affinities with the 

 Anomoura, and probably when we are in possession of fuller 

 information, we shall be able to refer it with confidence to the 

 Honiolad(B, a true Anomourous family. In the meantime we must 

 ask our readers to be contented with this brief description, in the 

 hope that more perfect materials may shortly be discovered. 



EXPLANATION OF PLATE I., Fig. 2. 

 Fig. 2. Prosopon mammillatum, H. "Woodw. Great Oolite, Stonesfield, twice the 



natural size. 

 Fig. 2a. Side view of same. 



From the cabinet of G. Griflath, Esq., M.A., Harrow. 



III. — On the "Lingula Flags," or '* Festiniog Group" 



OF THE DOLGELLY DISTRICT. PaRT III. 

 By Thomas Belt, F.G.S. 

 [PLATE IL] 



HAYING- described in some detail, the rocks of the Maentwrog, 

 Festiniog, and Dolgelly groups, and given a list of the fossils 

 found in each,^ I shall now make some general remarks upon the fauna. 

 In Part I. I have mentioned that out of all the numerous Trilobites 

 from the Lower Cambrian^ rocks, only two genera, Agnostus and Co- 

 nocoryphe, have been found in higher strata. Strictly speaking, this 

 is only true of Agnostus, which passes unchanged in type from the 

 Menevian to the Caradoc strata. On the contrary, the species of 

 Conocoryphe from the Lower Cambrian rocks are of essentially 

 different type from those that have been placed in that genus from 

 higher beds. The latter have, I believe, been referred to the genus Co- 

 nocoryphe rather from superficial resemblances than from true affinity. 



The possession of facetted or unfacetted pleurae has been con- 

 sidered of sufficient importance to separate closely allied species into 

 distinct families, and McCoy, in his Classification of the Trilohita, 

 has even founded his two main divisions upon that feature. The 

 facetting of the pleurae may be of generic, but it is not of higher 

 value. The absence of facets to the pleurae of Paradoxides and Oienu^ 

 does not prove, as some palaeontologists have supposed, that thej^ 

 were incapable of rolling up. Trilobites with flat pleurae required 

 no facets to facilitate that operation. It is only when the pleurae, 

 are bent down near the middle that the facets are of use in allowing 

 the ends of the pleurae to pack in underneath each other. 



It is through exaggerating the importance of this feature that some 



* See Geological Magazine, 1867, Vol. IV., pp. 493 and 536. 



* The Lower Cambrian period, according to the classification adopted in this paper 

 is only part of the " Primordial Zone" of Barrande, as that illustrious palaeontologist 

 has included in it Olenus and Feltura, which in reality helong to a second fauna. 



