1904.] FHOM EAST AFRICA AND ZANZIBAR. 383 



The following notes on the principal divergencies presented have 

 no pretension to be exhaustive, but may perhaps prove useful. 



1. As a rule the shape is somewhat high, elongated, and limaci- 

 form, with a narrow mantle-edge, but some species are low and 

 distinctly oval, with the mantle-edge very wide and ample. )Such 

 are Chr. reticulata, sykesi, cavce, and annulata hei'e described ; 

 and it would appear from the published plates and descriptions 

 that Chr. albescens, iheringi, punctikicens, histrio, propinquata, 

 splendens, albo-pusttdosa have a similar shape. Some species (e. g. 

 Chr. vicina) are capable of assuming two forms — one high and 

 narrow, the other flat and oval ; so the distinction in shape may 

 perhaps not be absolute *. 



2. The skin is usually soft and smooth, but the dorsal surface 

 bears tubercles in Chr. orsinii, sannio, pustulans, verrucosa, 

 lapinigensis, and the somewhat doubtful roseopicta of Veri'ill. 

 The very abnormal Chr. scahriuscula is spiculate with hard lumps. 



3. Chr. ritncinata, 2^<^'>^tharella, sannio, picttirata, camaena, 

 elegans, glauca, californensis, gonatophora, sycilla, have small 

 knobs, apparently of a glandular character, on the underside of 

 the mantle-margin. 



4. The colour is hardly ever uniform. There is usually a 

 coloured border (sometimes double) round the mantle- edge, and 

 generally a pattern on the back formed of stripes or spots. 

 Although this pattern may vary considerably within the species, 

 the spotted and striped forms appear to be distinct. Sometimes, 

 however {e. g. in Chr. runcinata), spots arranged in a line unite 

 to form a stripe, and Chr. semperi and nigrostriata appear to be 

 the same, except that the first is spotted and the second stiiped. 

 It would be rash in the present state of our knowledge to make 

 any general statement as to the correspondence between these two 

 types of pattern and other chai-acters, but in a considerable 

 number of species stripes are combined with an elongate form and 

 bifid teeth, with or without accessory denticles under the bifvirca- 

 tion (e. g., Chr. cceridea, gracilis, messinensis, sycilla, carnea, 

 hilai'is, lineata, marenzelleri, thalassopora, lajnnigensis) ; while 

 another combination, of an oval form, spotted pattern, and teeth 

 bearing many denticles but not bifid, is presented by Chr. reticu- 

 lata, sykesi, cava;, annidata, punctilucens, sjjlendens, and histrio. 

 Elongated spotted forms are not uncommon, but none of the oval 

 forms with ample mantles as yet recorded are striped. 



5. The branchiae range from 5 to neai-ly 30 in number, and are 

 variously arranged in a complete circle, or a circle more or less 

 open behind or a double spiral (see Plates XXIII. figs. 2 & 8 ; 

 XXIV. fig. 2). Sometimes the plumes are uniform in size ; some- 

 times those in front (more rarely those behind) are larger. Typically 

 they are quite simple, but frequently some are bifid and sometimes 

 several branches are developed (e. g. Chr. tryoni and striatella). But 

 when this occurs the ramifications are thin and irregular, and 



* [I observed this change of shape in some of the species here dealt with during 

 life.— C. C] 



