'812 PROF. G. C. BOURSE ON THE [NoV. 17, 



coneliologists assume, is found fossil from the Liassic onwai'd, 

 and is most numerous in species in the Miocene and Pliocene. 

 The subgenus Neritodonta Brusina, from the Tertiary of Dalmatia., 

 is of special interest, as being possibly a forerunner of the pul- 

 monate Hydrocena, now living in the same region. The genus 

 JS^eritoma Morris is found in the Jurassic of Europe, and the 

 ^subgenera JS/eridomns Morris and Lycett and Onchochilus Petho 

 in the Oolite and the Triassic and Jurassic respectively. The 

 genus Deianira Stoliczka is from the lacustrine deposits of 

 the Cretaceous of Europe, and Velates Montfort from the Tei-- 

 tiaries of Europe, India, and Madagascar. The limpet-like 

 •Pileolus Sowerby, resembling the modern Septaria, dates from 

 the Jurassic and Cretaceous. The Neritopsidje, differing from 

 the Neritidfe in the characters of radula and opercuhim, are 

 represented by a single recent species, Neritopsls radula, from 

 the E. Indies and Polynesia, but are fossil from the Secondaries 

 and Tertiaries. Since, with the exception of Deshayesia, thei-e is 

 no doubt as to the relationsliip of these extinct genera, it is clear 

 i:hat even in earlier Secondary times the Neritacea were dif- 

 ferentiated into marine, estuarine, and freshwater forms more or 

 less resembling those of the present day, and must have been 

 derived from an earlier stock, which we may look for in primary 

 -formations. But the remains of Neritacea from palaeozoic deposits 

 are at the best doubtful. As for some species of Nerita which 

 •have been described from this period, it is only necessary to 

 quote von Martens (27) : " Einige angebliche Arten von Nerita 

 aus den palaeozoischen Formationen sind betrefFs der Gattung 

 hochst zweifelhaft, wie es iiberhaupt meist eine unsichere und 

 'hoffnungslose Sache ist, palfeozoische Gastropoden auf Ga,ttungen 

 ■der Gegenwart zu beziehen." 



The family Maclureida?, of which Maclurea Lesueur, from the 

 Cambrian and Silurian of N. America and Scotland, is the sole 

 genus, is placed near the NeiitidcB because of its opercular 

 apophyses, but its affinities are very doubtful, and it has at 

 various times been placed in the Solariidte, Atlantidpe, Pleuro- 

 tomariida;, or between the Bellerophontidi© and Haliotidie. 

 JVaticopsis M'Coy, ranging from the Devonian to the Trias, and 

 the subgenus Trachydomia Meek and M'^orthen, from the Car- 

 boniferous, are placed in the Neritopsida? Ijecause of the characters 

 •of the operculum, but the shell is more like that of the Naticidee. 

 If these palaeozoic genera are really allied to the Neritacea, the 

 latter group is of great antiquity. On the other hand, the most 

 ■specialized of all the Neritacea, the pulmonate Helicinidfe, 

 Hydrocenida?, and Proserpinidfe, are only found in late tertiary 

 deposits, and have clearly been evolved in comparatively recent 

 time from Neritiform ancestors. 



The sole exception to this statement is furnished by the genus 

 Daivsoniella from the Carboniferous of Illinois. It is found in 

 association Avitli shells of the genus Piq^a, and there can be no 

 •>doubt that it was of terrestrial habit. Formerly placed in the 



