130 , Br. R. H. Traquair on the 



already so well known from the figures of Pander and Hugh 

 Miller that I need hardly enter into detail regarding them, 

 especially as I have in PI. X. fig. 3 accurately given their 

 respective shape and mode of overlap. They are six in num- 

 ber : — anterior median ventral (a. m. v.), posterior median 

 ventral (p.m. v.), two anterior ventrolaterals (a. v. I.), and 

 two posterior ventrolaterals {p. v. I.). I may, however, men- 

 tion that, judging from the course of the lateral-line groove 

 on the anterior ventro-lateral plate, Pander has reversed its 

 position, putting the front end behind and vice versd ; for 

 we shall presently see that on this plate the sensory canal 

 occurs on the anterior and not on the posterior part of its 

 surface. 



Distribution of the Lateral-line Grooves. — The course of 

 the lateral sensory canal is indicated on certain of the dermal 

 bones by conspicuous grooves, which, as in the case of Pter- 

 ichthys and Bothriolepis, have often been mistaken for 

 sutures. There is, however, no difficulty in distinguishing 

 them from sutures, when one by experience really comes to 

 know the characteristic appearance of the latter. 



On the anterior half of each anterior ventro-lateral plate is 

 seen a curved groove, starting from near the middle of the 

 anterior margin and then curving sharply round to proceed 

 to the inner border close behind the antero-internal angle. 

 On the median dorsal plate a groove is seen of a V-shaped 

 contour, the apex being in the middle line somewhat in front 

 of the posterior extremity of the bone, the limbs diverging 

 forwards towards the superior margin of the posterior dorso- 

 lateral plate. On the anterior dorso-lateral plate a continua- 

 tion of this groove runs forwards to the postero-internal angle 

 of the external occipital, near which it is met by a branch 

 coming diagonally upwards and forwards from the postero- 

 inferior angle of the plate. The side-canal thus formed passes 

 now on to the cranial shield at the point indicated, and there 

 at once gives off a branch running forwards and slightly 

 inwards, parallel with and close to the outer margin of the 

 median occipital, becoming lost on the posterior margin of 

 the central. The main groove then runs forwards and out- 

 wards parallel with the outer margin of the shield, giving off 

 first a branch passing to the external projecting angle of the 

 marginal plate, then turning forwards and inwards still 

 parallel to the shield-margin it passes on to the postorbital 

 plate, where it gives off another branch to the postorbital angle. 

 Here it bends sharply backwards and inwards at an acute 

 angle, runs on to the central plate, approaching its fellow of 

 the opposite side, and near the middle of this plate it again 



