96 BRITISH CAMBRIAN TRILOBITES. 



Moberg and Moller 1 have described a number of species which the}' refer to 

 Acerocare. They recognise the affinities of the genus to Peltura and group the 

 two together under the name of Peltura sensu lato, but divide this into Pelt urn 

 sensu stricto, with spines to the tail, and Acerocare, without spines or with only 

 rudimentary spines. According to them, the only practical difference between 

 Linnarsson's Cyclognathus and Acerocare lies in the size of the tail, and amongst 

 the new species which they describe there are intermediate forms. They, therefore, 

 include Cyclognathus in Acerocare. 



There can be no doubt that all these forms are very nearly allied. In all of 

 them the eyes are set far forwards and near to the glabella, and in all the genal 

 angles are rounded and either without spines or with only short and slender points. 

 The whole series might very well be included under the name Peltura sensu lato, 

 but to subdivide it according to the presence or absence of spines to the tail does 

 not lead to a satisfactory grouping. According to the classification of Moberg and 

 Moller, Brogger's Cyclognathus costatus would fall under Acerocare ; but in the 

 width of the axis, the shape and length of the glabella, the forward position of 

 the eyes, the pointed ends of the pleurae, and even in the form of the tail, it is 

 much closer to Peltura scarabosoides than to Acerocare ecorne. In fact, the only 

 respect in which it resembles the latter moi-e closely is in the absence of .spines 

 to the tail. I therefore place this, and the other forms referred by Brogger to 

 Cyclognathus, in Peltura rather than in Acerocare. And 1 should prefer to 

 distinguish between Peltura and Acerocare as follows : 



In Peltura the glabella is broad and reaches very nearly to the anterior margin, 

 in Acerocare it is somewhat narrower, and is separated from the anterior margin 

 by a distinct space. In Peltura the eyes are set farther forwards than in Acerocare. 

 In Peltura the thoracic axis is wider than the pleurae, and the pleurae are obliquely 

 truncate, but end in short points; in Acerocare the axis is narrower than the 

 pleurae, and the ends of the pleurae are usually blunt, but may end in short points. 

 In Peltura the tail is small, wider than long, with a broad axis terminating bluntly, 

 and the margin maybe either entire or spinose; in Acerocare the tail is larger, 

 and longer in proportion to its width, and the axis is narrower and more pointed. 



To judge from the descriptions and figures given by Linnarsson and by Moberg 

 and Moller, the tail of Cyclognathus micropygit.s, Lnrsn., is distinctly different from 

 that of either Peltura or Acerocare. The axis is deeply divided, so that in Moberg 

 and Moller's figures it appears as a series of isolated rings, and it dies away 

 posteriorly without any definite termination. The lateral lobes are strongly bent 

 down, without spines or marginal fold, and are marked by a series of fine striations. 

 Acerocare granulatum, Moberg and Moller,' 2 has a similar tail. In both these forms 



1 Om Acerocarezonen : Geol. F6ren. Stockh. Forh., vol. xx (1898), p. li»7. 



2 Lne. cit., p. 244, pi. xi, figs. 1-9, pi. xiv, figs. 1-5. S^e also Supplement till ' Om Acerocare- 

 zonen,' loc. cit., p. 315, pi. xvi, fig. 6. It should be observed that the specimens figured on pi. xiv as 



