part 4] THE EVOLUTION or liassic aASTBOPODs. 325 



Loxonema, known only in Palaeozoic and Triassic rocks, and 

 presumably ancestral to the other genera, is ornamented by 

 sinuous stria?, sometimes with spirals and more rarely with axial 

 ribs ; the genus Zygopleiira, which occurs in Liassic rocks, has 

 curved axial ribs without spiral ornament: while the section Kafo- 

 sira, which Ave propose to treat as a genus, resembles Zygopleiira^ 

 except that it has spirals ; Hypsipleura has flat whorls and straight 

 axial ribs. The genus Anoptychia proposed by Koken is taken by 

 Cossmann to include species with flat whorls ornamented by ribs 

 (in the early stages), which become obsolete on the last whorls ; it 

 appears to us that this genus may include homoeomorphs of dif- 

 ferent stocks. The genus Rigauxia, as interpreted by Cossmann 

 (3, pp. 206 ef seqq.) has been made to include two distinct types : 



(a) Species with a liigh sutural angle, deep narrow sutures, and a flat or 

 even concave whorl, generally smooth or nearly so. 



Q)) Species with numerous spirals, widely-spaced oblique folds, and imbri- 

 cate whorls with shallow concave sutures. 



The species of the former group, of which the genotype 

 Higauxia canaliculata is a member, are mainly found in post- 

 Liassic rocks, while those of the latter group appear in Liassic 

 rocks. These species do not seem to us to be so clearly related to 

 the H. ccDialiculafa type as they are to certain species of Kcitosira, 

 from which they appear to have been evolved, and we propose to 

 group them with that genus. 



We classify Liassic Loxonematidse, briefly, as follows : — 



Zygopleura : whorls convex, sinuous axials, no spirals. 

 Katosira : resembling Zygoi')leura, bixt spirals are present. 

 Hypsipl&ura : whorls flat, straight axial ribs. 



It appears that Zygopleura has evolved from the smooth 

 Loxonema in pre-Liassic times; Katosira may be regarded 

 as a further development, probably from some form of Zygo- 

 pleura ; Avhile Kypsipleura may represent a similar but parallel 

 development. 



This family exhibits the various stages of growth and decline 

 that we have recognized in the Procerithidse ; but, while there are 

 catagenetic forms with diminished ornament, there are no traces of 

 contraction of the Avhorl towards the base, such as are observed in 

 ' JExelissa.'' On the other hand, some species of both Katosira and 

 Zygopleura show a tendenc}" in later stages to mould the posterior 

 portion of the later Avhorls on the preceding whorls : that is, the 

 whorls become imbricate. 



(A) ZraoPLEUKA Koken. 



Probabl}^ represents an old genus evolved from Loxonema by 

 the development of ribs. According to Dr. Cossmann, Zygoplenra is 

 not found higher than the Lias; it is extremely common in English 

 Charmouthian rocks, but ver}^ few forms are obtained in higher 

 formations. The earliest species of Zygopleura that we have 

 studied ; namely, those from the Ehaetic and Hettangian, are 



