224: Geological Society, 



the affinities of Qlossopterls, which the Author is disposed to in- 

 clude in the CycadofiUcales. He is of opinion that the anatomical 

 features of the seeds, so far as they can be made out from the 

 impressions, favour their inclusion in the Trigonocarpales. 



3. ' The Evolution of Certain Liassic Gastropods, with special 

 reference to their Use in Stratigraphy.' By Miss Agnes Irene 

 McDonald, B.Sc, and Arthur Elijah Trueman, D.Sc, F.G.S. 



The gastropods dealt with in this paper are turriculate foi-ms, 

 which have generally been called 



(a) Cerithiiim, wkich includes those Liassic species that are ornamented 

 with axial and spiral threads, forming a network, often with tubercles ; now 

 referred to the family Procerithidaj, Cossmann. 



(b) Chemnitsia, which includes species ornamented by strong axial ribs ; 

 now referred to the family Loxonematid^, Koken. 



An endeavour has been made to study these gastropods in t]\e 

 light of modern palseontological research, and suggestions for their 

 classification, based on ontogenetic and other evidence, are made. 

 The position and characters of the ornamentation have proved of 

 value in classification, when taken in conjunction with the other 

 characters of the shell. 



Many of the biological principles that have been studied in 

 such groups as the Ammonites are clearly illustrated by these 

 gastropods. In numerous series, acceleration and retardation of 

 development is indicated. Examples of homoeomorphy of several 

 t^pes have been noted ; the recognition of such homoeomorphs, 

 which often occur at different horizons, is essential in the identifi- 

 cation of species in these groups, if they are to be of value in 

 correlation. 



The Procerithidse of the Lower Lias are chiefl}^ species of Pro- 

 ceritlihim, in which the flattish whorls have reticulate ornament 

 based on three spirals ; this central stock is also common in the 

 Inferior Oolite, where it is represented by similar species with four 

 spirals {Cerithiiim muricatum). This series probably gave rise to 

 many recent Cerithidse which have more than four spirals. 

 Besides the species with three spirals, there are in the Lias many 

 forms which the Authors regard as more specialized, and are charac- 

 teristic of particular horizons. Other genera of ProcerithidiB are 

 recognized, of which Cerithinella and Paracerithium have dis- 

 tinctive ornament. The pupoid forms which have been grouped in 

 the genus Exelissa are regarded by the Authors as catagenetic 

 descendants of diverse species of Procerithivm. 



The Loxonematidse of the British Lias are of two types— one 

 with axial ornament only {Zyr/opJeura), the other witli axials 

 and feeble spirals {Katosira). Each of these genera during the 

 Lias evinces a tendency to increase the number and curve of the 

 axials. In development, axials always appear before spirals 

 among the Loxonematidte, while spirals are developed first among 

 the Procerithidse. 



