50 G. LINDSTRÖM, (IN TIIE SILURIAN GASTROPODA AND PTEROPODA OF GOTLAND. 



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

 instance compare the rostra of the genus Scalpellum Darwin, Lepa,dida3 pl. VI, tigs. 

 6a, 7a, 8a, and arnongst the fossil ones Pollicipes Nilssoni Darwin, Fossil Lepadida;, 

 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 raanner. In the Chitonida) and consequently also in Chelodes its superior 

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

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

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

 stränge if only one valve, the regularly formed rostrura, 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 »that 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 dibranchiate Cephalopoda». He continues, that if they are de- 

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

 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 difife- 

 rent orders, 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 mäss 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 

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

 is the circumstance that the exteriör 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 palasozoic Chitons, also wanting apophyses, have been found in their ori- 

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

 ninck's »Deux espéces 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, Avhere 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 with Chelodes that both may be con- 



') Darwin Monograph of Cirrhipetlia. LepadidiB. Eay Soo. 



-) Die Aptyclien als Beweismittel fiir die Dibranchintennatur der Ammoiiiten. N. Jalivb. fiir Mineralogie 

 etc. 1881, 1 Bd ?Ieft 1, p. 44. 



^) Doouments pour servir å Fétudc des Faunes dévoniennes dans Touest de Prance in Mém. Soc. Géol. 

 de France 3:me Sér. torae II, p. 15, pl. II, fig. 3, 3 a, 3 b. 



