404 Gcoloyical Society. 



IMIOCKE DINGS OF LKAllXED SOCIKTIES. 



GEOLOGICAL SOCIETY. 



November 2i>th, 1901.— J. J. H. Teall, Esq., :\[.A., V.lMl.S., 

 President, iu the Chair. 



The following communication was read : — 



' Notes on the Genus Lichas.' By Frederick Ilioliard Cowpor 

 Heed, Esq., M.A., F.G.S. 



The various attempts which have been made to separate this 

 genus into subgeneric groups have not been alto;;other satisfactory, 

 owing to the difficulty of deciding what are the more important 

 structural features. A natural classification, as opposed to an 

 artiKcial one, should be based on the structural characters of the 

 head-shield ; and the variations in the form and lobation of the 

 glabella in the Lichadidaj (as Beecher has remarked) indicate dif- 

 ferences in the relative development of the appendages and organs 

 of the head. The origin and application of the various proposals 

 for classification are considered, and certain synonyms and names 

 of prior application to other organisms are rejected. Next, the 

 characters of the original type-species of the various subgenera are 

 summarized. The second part of the paper contains a critical con- 

 sideration of the homologies of the furrows and lobes in the glal)ella 

 of the I.ichadida;. Following Beecher's scheme, the anterior lateral 

 portion of the so-called median or frontal lobe is considered to 

 correspond to the anteunulary or true first lo])c of the glal)ella. 

 The so-called ' first ' lateral lobes of Lichas would correspond to the 

 fused antennary and mandibular lobes, the true second and third 

 lobes of the glabella. The lateral lobes, which are usually termed 

 the ' middle ' or ' second ' lateral lobes, become homologous with 

 the fourth or first maxillary ; and the neck or occipital lobo or ring 

 falls into its right place as the second maxillary lobe. By means 

 of this principle an attempt is made to discover the principal lines 

 of moflification along which the evolution of the head-shields of 

 the LichadidiC has proceeded. In the third i)art of the paper the 

 Lichadidic are divided into two great groups : (1) tliat with a pair 

 of bi-composite lateral lobes to the glabella and a nu)ro or less 

 definite fourth pair of lateral lobes; and (2) a group with a jiair of 

 tri-composite lateral lohes, through the fusion of the fourth pair with 

 the bi-composite pair of the preceding group. Names are pro|)osed 

 for each group, and also, where necessary, for the eight sections, of 

 subgeneric value, into which each group is subdivided. The paper 

 closes with a list of the British members of the family Lichadidai, to 

 show their distribution among the groups and sections. 



