50 G. LINDSTROM, ON THE SILURIAN GASTROPODA AND PTEROPODA OF GOTLAND. 



tween the valve which Darwin calls rostrum ') and Chelodes. This is evident if we for 

 instance compare the rostra of the genus Scalpellum Darwin, Lepadidai pi. VI, figs. 

 Ga, 7a, 8a, and amongst the fossil ones Pollicipes Nilssoni Darwin, Fossil Lepadidte, 

 PL III fig. lid, and it seems indeed very likely that some of them really have been 

 mistaken for Chitons and figured by Kirkby in the paper cited above. The general 

 outline is almost the same and on the inside there is also an apical area, composed 

 of concentric lines of growth. This area seems, however, not to have been formed in 

 the same manner. In the Chitonida3 and consequently also in Chelodes its superior 

 or youngest margin is always elevated above the upper or distal part of the interior 

 face, as is also the whole area; in the Lepadida) again the area is generally lower than 

 the other surface. If Chelodes were a Cirrhipedian, it would be in the highest degree 

 strange if only one valve, the regularly formed rostrum, had been preserved and not 

 a single one of the others, which are at least six in number, but in some species much 

 above hundred. 



A quite different conjecture as to the nature of this fossil is given by Ihering in 

 his paper on Aptychus"). He there, page 70, says Mthat it is at least to me highly 

 probable that what Barrande has described as the plates of Chitons, in the reality are 

 Aptychs of Silurian dibraiichiate Cephalopoda". He continues, that if they are de- 

 rived from Chitons, they can be interpretated only as the final plates and it is strange 

 if only these were preserved of all eight plates. Against this is to be remarked first 

 that the plates figured both by Barrande and by me are not all identical, but of diffe- 

 rent ordei's, secondly that in the recent Chiton hastatus the plates are quite as acu- 

 minate and elongated relatively as in the Silurian ones. Moreover, the conformation 

 of the inside, which was unknown to Ihering, removes these fossils from Aptychus. 

 As to the microscopical structure, on which Ihering justly lays so much stress, there 

 is unfortunately no guidance to be had, as the chief mass of the very thick plates of 

 Chelodes has been converted into clear, transparent calcareous spar. 



What, for the rest, adduces me to range Chelodes, at least provisionally, with the 

 Chitonidai, notwithstanding all that has been said to prove its similarity with other groups, 

 is the circumstance that the exterior ornamentation of the plates is in complete accordance 

 with that of the Chitonida?. Moreover, there is at least one ascertained instance, in which 

 valves of the palaeozoic Chitons, also wanting apophyses, have been found in their ori- 

 ffinal position. In the specimen of Chiton Grayanus, drawn on plate I fig. 1 of de Ko- 

 ninck's »Deux especes siluriennes de Chiton)), there are five valves in juxtaposition. 



But if we now are to conclude that Chelodes is one of the Chitonids, it shows 

 so great differences from the others, that it cannot belong to the genus Chiton pro- 

 per, where Barrande placed it, but must form a genus of its own. Of all the palajo- 

 zoic subgenera, no less than 13 created within the last twenty years,there is one, Sag- 

 maplaxus Oehlert^), which so nearly coincides Avith Chelodes that both may be con- 



') Darwin Monograpli of Cinliipedin. Lepadidre. Ray Soc. 



•*) Die Aptyclien als Bewcismittel fiir die Dibranrliiatennatur der Aminoniten. N. .Talirb. fiir Mincralogio 

 etc. 1881, 1 Rd Ilcl't 1, p. 44. 



^) Documents pour servir h I'utudc dos Faunes devoniennes dans I'ouest de France in Mum. Soc. Geol. 

 de France 3:rae Ser. tome II, p. 1.''), pi. II, fig. .3, 3 a, 3 b. 



