2b PKOCKEDINGS OF THE 



That SDine Anaspida survived unchanged until the end of 

 Devonian times seems to be ])roved by hhiplianerups lowfirvas from 

 tbe Upper Devonian of Canada *. Tliey must, however, have 

 been very rare or h)ral, for hitlu-rt(j they are iuiown oidy by one 

 specimen of the species just mentioned. 



Hktekostuaci. 



The next order of Ostrueoderms comprises famibes adapted for 

 bfe n)aiidy at I be botloui of the shores, estuaries, or lakes in 

 which, they dwelt. 'I'bey have a relatively large depresseil head, 

 and a small mobile tail ending in a heterocej'cal tin. Most of 

 tbem are larger than the Anaspida, and they range upwards 

 tbrougb tbe Devoinan fornuitious, increasing in size until in tbe 

 L [iper Devonian some of them (Psuiiimosteus) are more than half 

 a metre in length f. These are the largest Ostracoderms linown. 



Fis:. 1. 



Restoration of I'helodus scoficiis, from tho l^owntoiiian Passage Eeds 

 of Laiiarksliire, about one-liall' iiat. size. Tlie liead shown IVoiti jibove, 

 the tail twisted to be seen mainly in side-view. (After Traqwair.) 



Tbe most abundant early members of the order in tbe Down- 

 toniaii Passage Beds and tbe Up]jer tSilurinn are protected only 

 bv a nearly uniform covering of shagreen closel)^ similar to that 

 of sharks. Wben tbe isolated dermal tubercles of T/nlodus were 

 tirst discovered, indeed, they were regarded as belonging to 

 Elasmobranch fishes; but it is now clear that this simplest type 

 of vertebrate armature was also assumed by primitive tisb-like 

 organisms Avhicb show no other real resemblance to Elasnio- 

 brancbs. As described by Traquair +, Thclodits (tig. 1) anil 

 Laiutrhia, when crushed in their fossilised state, exbibit a broad 

 head-region, rounded in front and truncated at tbe hinder bonier, 

 from the middle of which tbe comparatively slender body-region 



* A. S. Woodward, Ann. & Maj;. Nat. Hist. \1\ vol. v. (lUOO), p. 410, pi. 10. 



t A. S. Woodward, Ann. & -Mag. Nat. Jfist. [8] vol. riii. (1911), p. 649, 

 ])1. U. For niid'o. slriu'tiire sec J. Ivia-r, Rep. 2nd Norweg. Kxped. ' Fram ' 

 18i»8-l<.l(ll2, no. ;53 (191.")), pp. 1.'4. -^8, :>;">. pi. ;5. (igs. .'i, 4, jil. li. text-ligs. a. 8. 



X R. 11. Traquair, Trans. Roy. Soc. Edinb. vol. sxxix. (1899), pp. 8-J9-So4, 

 pis. 1-4 ; and loc. cit. vol. xl. (1905), pp. 880-883, text-figs. 1, 2, 



