542 PKOF, E. W. CLAYPOLE ON THE STKUCTtrRB 



40. On the Steucture of the American Ptbeaspidian, Pal^aspis 

 (Clatpole) ; with Eemarks on the Family. By Prof. E. W. 

 Claypole, B.A., D.Sc. (Lond.), P.G.S. (Eead June 22nd, 1892.) 



Contents. 



Page 



I. Eeview of the Discovery of Pafeas^is 542 



II. Kunth's Discovery of Fteraspis Integra 543 



III. Controversy and Results.— Ifo^flfs^zs, Lankester 645 



IV. Discovery of -D^23/asp^s (Matthew) in Canada 546 



Y. Von Alth's Specimen and Interpretation 548 



VI. The two Shields of Pa^«as2?2s 549 



VII. The Lateral Plates oi Palceaspis 551 



VIII. The Lateral Organs, Tins,' of Palceaspis 553 



IX. Comparison of P«^<s<2s^«s with other Pteraspids 557 



X. Attempted Restoration of Palcsaspis 559 



XI. Amended Definition of Pafeas2?zs 561 



I. Eeyiew oe the Discovery op Pal^aspis. 



In the volume of this Journal for 1885 ^ the present writer 

 published a description of the first specimens of Pteraspidian fishes 

 that had come to light in the JS'ew "World. They were discovered 

 in the autumn of 1883 in a bed of variegated marl, and later in a 

 bed of red and green sandstone enclosed in the marl and belonging 

 to the Onondaga or Salina Group of the Upper Silurian formation. 

 The fossils were found in Perry Co., Pennsylvania, in the survey of 

 which the writer was at that time engaged. The sandstone and 

 shale pass under the town of New Bloomfield, and for this reason the 

 stratum has received the name of the ' Bloomfield Sandstone.' 

 The horizon of these beds corresponds with that of the English 

 Lower Ludlow, or with the interval between that and the Wenlock, 

 inasmuch as the Lower Helderberg Limestone which overlies it is 

 unquestionably the palaeontological equivalent of the English 

 Ludlow, and the Niagara Limestone which lies next beneath it is 

 as certainly the counterpart of the English Wenlock. The whole 

 mass of the Salina Shale is, in Perry Co., about 1500 feet in thick- 

 ness, and for the most part, with the exception now noted, is 

 entirely unfossiliferous. This mass is but slightly represented in 

 tlie English series. 



The fossils consisted of the usual shields resembling those so 

 long known from the rocks of nearly corresponding age in Here- 

 fordshire, and occurred in great abundance, but were for the most 

 part broken or in the form of casts. Good representative shields 

 are very scarce, partly no doubt because all the material accessible 

 on the several exposures was considerably weathered and deep 

 quarrying was not practicable. 



Two forms were observed among these fossils, and they were 



' Vol.xli. pp. 48-63. 



