Oct.. 

 1891. 



FORE-RUNNERS OF BACKBONED ANIMALS. 597 



principal types of life worthy of special notice. There is a limbless, 

 hard endoskeleton in the Lower Old Red Sandstone ; there is a 

 simple shielded organism, with no bony tissue in its armour, in the 

 Upper Silurian and Lower Old Red Sandstone ; a similar shielded 

 organism, covered with few plates, but with some bone-tissue in their 

 substance, occurs in the same formations ; and a more advanced 

 armoured creature, with a complex of symmetrically arranged bony 

 plates, is met with both in the Upper and Lower Old Red Sandstone, 

 but never earlier. 



The limbless skeleton from the Lower Old Red Sandstone of 

 Caithness was briefly noticed two years ago by Dr. R. H. Traquair 

 (9) under the name of Palcrospondyhis giinni ; and the writer is indebted 

 to Mr. Donald Calder of Thurso for a small series of specimens from 

 which the accompanying partial restoration (Fig. i) is made. It 

 belongs to a very small organism, rarely, if ever, exceeding two inches 

 in length. There is a well-formed calcified cartilaginous skull, with 

 a lateral extension on each side at the position of the organ of hearing 

 {an.), but with no certain evidence of a pair of nasal capsules ; the 

 region of the nose, indeed, is so narrow as to suggest some comparison 

 with the ordinary monorhine lampreys and hag-fishes. In front 

 there is a pair of processes (/>.), which may be of the same nature as 

 those named prepalatine by W. K. Parker, or may be merely lip- 

 cartilages ; a lateral pair of minute points further recalls the lip- 

 cartilages, and a great broad ring (/.) crushed upon the anterior part 

 of the skull, bears an extraordinary resemblance to the annular 

 cartilage forming the rim of the mouth in the lamprey. The back 

 of the head is well-defined, and immediately adjoining it posteriorly, 

 there is a hard shield (5.) presumably covering part of the back of the 

 animal. The notochord is surrounded by a series of calcified rings 

 {:'.) developed in its sheath, apparently with short intervening spaces; 

 and towards the end of the tail there are indications of delicate 

 neural and haemal spines. 



Such is the sole information at present available in regard to 

 Palcrospondylus ; and this, it will be observed, merely suffices to place 

 the animal at .the base of the backboned series. It seems to possess 

 an unpaired nose, lip-cartilages in place of functional jaws, and no 

 paired limbs ; thus agreeing precisely with the lampreys and hag- 

 fishes, of which the fossil representatives have long been sought. 

 Theoretically speaking, allies of these limbless Chordata ought to 

 have been a dominant type in the late Silurian and early Old Red 

 Sandstone age ; and it is a well-known law that animals, when 

 dominant, attain to a higher degree of development than at any other 

 time in their history. It is extremely probable, therefore, that 

 Palaospondylus belongs to this interesting category. 



While there is much probability that the skeleton just described 

 can be paralleled among existing animals, there is no possibility of 

 recognising any living congeners of the armour-plated organisms of 



