PROFESSOR OWEN ON LIMULUS POLYPHEMUS. 199 



concentrative unity, is exemplified in the living representative of the old form 

 discovered by Salter in the bed of a Silurian sea now contributing to form the county of 

 Shropshire (see woodcut of Hemiaspis, reproduced, fig. 72, p. 200). 



" The ancestral ' pleon ' [abdomen) has been almost ' rubbed out ' in the thousand- 

 fold generations of which the Salem King-crab is the heir ; but the palaeozoic taint sticks 

 to the nerve-element. Or shall we say that Limulus, made perfect for its sphere and 

 habits of life, must have its ' alpen-stock ' unbroken, of compact stuff without joints near 

 the grasped end ? But then the teleologist has to give an account of the intermediate 

 or ' evolutionary transitional ' condition of the three ' pleonal ' {abdominal) segments 

 manifest outwardly, as doubtless by their nerve-pairs and probably ganglion-centres 

 within, but soldered together or 'anchylosed' in Konig's and Baily's Bellinurus." 

 (Owen, op. cit., p. 501.) 



By the kindness of Mr. Robert Etheridge, Jr., F.G.S., Acting Palaeontologist on the 

 Geological Survey of Scotland, I have received from him for examination a ' telson ' or 

 tail-spine of Stylonurus, discovered by James Powrie, Esq., E.G.S., of Reswallie, Forfar- 

 shire ; in the grey micaceous, " Lower Old Red Sandstone " of the Sidlaw Hills, Forfar- 

 shire, which have already yielded to that ardent geologist such a grand series of Fish 

 and Crustacean remains (see Parts I and IV of this Monograph, &c). 



Mr. Etheridge at once detected the importance of this specimen, and obtained 

 permission of Prof. Geikie, F.R.S., the Director of the Geological Survey of Scotland, that 

 it should be placed in my hands for examination and description. It is figured in the 

 accompanying woodcut (Fig. 73, p. 200). 



The specimen measures nearly seven inches in length, and is slightly injured at each 

 extremity, which renders its original full length rather indeterminate, although from the 

 curvature of the lateral borders it would seem to be nearly entire. Its greatest breadth is 

 an inch and a half. The flat, central part is one inch in breadth. The borders are 

 raised on each side, the centre being flat ; but as we are looking at an intaglio impression 

 of the fossil, the original, no doubt, had the centre raised and the borders bent down- 

 wards. In form it differs from Stylonurus Scoticus, being more elongated and straighter, 

 and less spatulate in outline, agreeing better with S. ensiformis, H. W., from the 

 same locality, and also collected by Mr. Powrie (see Part IV of this Monograph, PL 

 XXI, fig. 5)". 



What renders this specimen of so much interest is the fact that it affords clear 

 evidence of segmentation, the central part being divided by ten transverse lines, which 

 extend from side to side, but do not cross the raised lateral border (save in the case of 

 the sixth and seventh from the proximal end, which slightly indent it). The border 

 of the telson is finely serrated on either side, but no scale-markings are visible. 



This specimen appears to lend important weight to Professor Owen's conclusion as to 

 the compound nature of the " telson " in Limulus polyp/iemus, and I have therefore felt 

 that its introduction here would prove both opportune and appropriate. 



