38 BRITISH CAMBRIAN TRILOBITES. 



On account of the differences between the exterior and interior of the test, and 

 between the young and the adult, there is considerable variety in the appearance 

 of different specimens, and but for the large series obtained from South Wales, 

 some of the variations due to age or to the mode of preservation might be supposed 

 to constitute distinct species. So far as I have observed, however, there is no 

 other species in Great Britain which has a narrow many-ringed axis to the tail and 

 unribbed lateral lobes. The head of M. sculptus, in the imperfect condition of the 

 only complete specimen known, is in general shape not unlike an imperfect head 

 of M. punctatus, but in the former species the margin shows a row of rather large 

 tubercles in front, which offers little resemblance to the fine crenulation on the 

 under surface of the margin in M. punctata*. The species from Comley are so 

 distinct that there is no possibility of confounding them with this form. 



In consequence partly of the variations here described, and partly of the 

 brevity of Salter's description and the inadequacy of his figures, forms which 

 appear to be identical with our M. punctata* have been recorded under other 

 names both in Scandinavia and in North America. 



The Swedish specimens described by Linnarsson as M. scanicus and M. eucentrus 

 appear to be the same form under different conditions of preservation, and seem 

 to be indistinguishable from the British species. M. scanicus is represented by 

 Linnarsson without a neck-spine, but he remarks that some indistinct impressions 

 seem to show indications of such a spine. In our own specimens the spine is often 

 lost, and the glabella appears to end bluntly. Linnarsson also states that there is 

 no definite ornamentation on the cheeks, and the margin of the head is smooth, 

 that the furrow in front of the glabella is narrower than in M. punctatus, and the 

 cheeks not so much contracted forwards. The punctation on the cheeks of 

 M. punctatus is generally somewhat indistinct, and often quite invisible, and in the 

 other characters mentioned Linnarsson's description agrees with the external 

 moulds of M. punctatus. Moreover, according to Gronwall the stone-casts of 

 M. scanicus show a weak crenulation of the margin. The other Swedish form, 

 described by Linnarsson as M. eucentrus, is believed by Gronwall to be merely a 

 time mutation of M. scanicus, and does not differ from the ordinary internal casts 

 of M. punctatus. 



The form described by Walcott in 1884 as M. punctatus is also probably 

 identical with the British species. It is true that the figures of the head which he 

 gives indicate considerable differences. The occipital spine is represented as very 

 short and obtuse, and the margin is ornamented with a row of rather distant 

 tubercles. In these respects, however, the figure scarcely agrees with the descrip- 

 tion ; for AValcott states that the occipital spine in medium-sized specimens is as 

 long as the glabella, and that the marginal groove is marked by numerous short 

 furrows at right angles to the margin. The description, indeed, agrees completely 

 with our Welsh specimens, and judging from it alone I should have little hesitation 



