KTTNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND. 59. NIO 3- 79 



at least, to group the species according to their sculpture, and such an arrangement 

 will, moreover, correspond to the lines that the development has followed. If we 

 study the sculpture of a Chama we will find that towards the umbones a concentric 

 ornamentation is predominant, even when a radiating sculpture is prevalent towards 

 the periphery. This fact appears very strikingly in Ch. fragum, where the nepionic 

 stage is followed by a shell with well developed, distant concentric lamellae; these 

 are only divided into spines låter on. This circumstance proves that a concentric 

 sculpture is the original one in Chama, just as has been shown, for example, in 

 Trigonia. We therefore seem justified in grouping the Chamas, according to the 

 stage which their sculpture represents, in a series showing a transition from a primi- 

 tive concentric to a secondarilv acquired radiating ornamentation. 



As the first group in this series we distinguish those forms of Chama where 

 concentric lamellae prevail throughout and predominate över the radiating features.* 

 The type of this group is Ch. lazarus, where the lamellae are rather distinct and 

 remote towards the umbones, but differentiate into irregular lobes or branched pro- 

 jections towards the margins. Within the same group I include Ch. pulchella Reeve 

 (the sinistral specimen; ef. Odhner 1917) as well as Reeve's Ch. praetexta and lobata, 

 which may perhaps be nothing but juvenile specimens of Ch. lazarus. 



In all other Chamas the concentric lamellae are either much closer or much 

 depressed; in both cases they are more or less subjected to a division into lobes, 

 scales or spines. Among these forms we can distinguish a group where a tendency 

 to form lobes may be observed; these lobes are generally localized into two radi- 

 ating lines from the umbones to the postero-inferior side, while in the frontal part 

 of the shell the lamellae are still continuous and undulatory. To this group are to 

 be referred Ch. frondosa, sinuosa, macerophyUa and senegalensis (= crenulata). 



In a third group no large lobes are seen but the lamellae are fringed with 

 small (flat) scales, about equal in size throughout the shell, or only slightly larger 

 on its posterior side without showing any tendency to be localized into a separate 

 zone. In this group, which embraces, for example, Ch. gryphoides and its allies (Ch. 

 circinnata and nicolloni), Ch. pellucida and pacijica, the concentric lamellae can be 

 still traced as the common base from which the flat scales project. In Ch. gry- 

 phoides the latter may be changed into spines (var. aculeata), but the traces of the 

 primary lamellae are still observable, thus presenting a difference from the Ch. 

 spinosa group. 



We now arrive at the forms of Chama in which the primarily erect con- 

 centric lamellae have been depressed and their primary undulations have been pro- 

 duced into isolated scales of a vaulted shape. Here we find some forms in which 

 a differentiation of the posterior scales has taken place, so that here there are many 

 rows of more elongated spines, while they are shorter in the foremost part; further, 

 along the postero-dorsal margin no scales appear. In this group I include Ch. im- 

 bricata, Ch. divaricata and its allies (brassica, rubea, dunkeri, ambigua, semipurpurata, 

 which may perhaps be only synonyms of the first-named species). 



