22 William Patten. 



Holaspis the presence of a semicircular series of minute grooves or pits. They differ, 

 however, from those of Tremaiaspis in their somewhat indefinite lineal arrangement, with the 

 grooves sometimes running across the series instead of parallel with it. Lankester's 

 figures of Pteraspis (PI. VI figs. 1 and 6) show two lines of dots that may perhaps represent 

 the marginal and the posterior dorsal line of Tremaiaspis. 



In Pterichthys and Bothriolepis, Traquair has described what he calls «a lateral line 

 system)) consisting of a series of grooves « which are apt to be and have often been 

 mistaken for sutures)). Traquair states, p. 488, that «they)) (the lines) «do not occur 

 on the inferior surface of the bones, and their connection with a similar groove running 

 along each side of the body amply demonstrates their true nature. » 



It seems to me that we must await further evidence before definitely accepting the 

 lines of surface markings in Tremaiaspis, Pieraspis and Pterichthys as evidence of the 

 presence of lateral line organs, directly homologous with those of modern vertebrates. The 

 absence of markings on the internal surface of the shell does not preclude the possibility 

 that the grooves in question are fused sutures; and it is not at all certain that the longi- 

 tudinal groove in Pterichthys is comparable with the lateral line on the trunk of true ver- 

 tebrates. 



There is however considerable resemblance between the lines of dots and dashes of 

 Tremaiaspis and the grooves of Pterichthys. The circum-orbital and the marginal lines of 

 the former being clearly the same as the so called trunk line and the circum- 

 orbital line of Pterichthys. There is even in Pterichthys, the slight backward bend or 

 loop at the anterior end of the circum-orbital line so strikingly shown in Tremaiaspis. 

 The transverse and the posterior dorsal lines of Tremaiaspis do not appear to be repre- 

 sented in Pterichthys. 



An examination of the two splendid heads of Tolypaspis preserved in the Petersburg 

 Academy shows the presence of a V-shaped ridge on the dorsal shield that appears to in- 

 dicate the presence of a circum-orbital line similar to that of Tremataspis. This ridge is 

 shown in Rohon's figures' 93 (PI. I. fig. 45), but is not otherwise referred to. In taking 

 an impression of one of these specimens, a part of the shell was removed showing the 

 presence of a median pit on the inner surface of the shell in the median eye region. It is 

 thus evident that the shield in question is comparable with the dorsal shield of Pteraspis, 

 not with the ventral one as Schmidt supposed. Rohon appears to have entertained the 

 same view as Schmidt in regard to it. Our interpretation is further confirmed by the 

 presence of the marginal notch and the circum-orbital line mentioned above. 



