LINNEAN SOCIETY OF LON^DON. ^^ 



on each side to two widely- separated areas, which have been 

 mistaken for marks of sense organs *. One I'emarkably j^reserved 

 specimen of Tremataspis exhibits a small vertebrate brain of 

 iilinost diagrammatic simplicity t; and several known shields of 

 this genus are pierced above the position of the auditory oi'gans 

 by a pair of foramina which may represent the ductus endo- 

 lymphiiticus of each side. In Treumtaspis and Didi/ruaspis some 

 ot' the anterior body-scales are fused nito a pliite both dorsally 

 and ventrally, and tbis armour is firmly united with the head- 

 shield. The extent of the dermal plates in these two genera is 

 therefore as great as in tha Heterostracan geneva. Drejninaspis and 

 r/il/Jlohjtis. 



The Osteostracans are connected witli the most primitive 

 Heterostracans, such as 2'helodiis, by Ateleaspis t, from the Upper 

 Silurian Passage Beds, in which the polygonal phites of the liead- 

 ■shield are not fused together into a rigid covering. Indeed, if 

 :geologists are not mistaken as to the age of the rocks in which 

 tliey occur, nearly similar polygonal plates are among the earliest 

 known fish-remains, even so ancient as the Ordovician period. 

 The plates named Astraspis desiderata §, said to be found in the 

 •()rdovici;in near Canyon City, Colorado, U.S.A.. are essentially 

 ■similar in microscopic structure to tiiose ot" the Osteostracans, 

 •except that \\\ej lack the laminated inner layer — the part of the 

 4irmour which theoretically would be latest in development. 



Antiakchi, 



The most highly specialised order of Ostracoderms occurs onlv 

 in Middle and Upper Devonian formations, and its ancestral 

 forms remain unknown. The head and the greater part of the 

 trunk are covered with plates symmetrically arranged according 

 to the xlisposition of the sensory canals ; and a pair of movable 

 lateral appendages, also encased in plates, is fitted anteriorly to 

 the armour of the trunk. The small tail, ending in a heterocercal 

 itin, is either scaly or naked. Bone-cells are present in all layers 

 of the armour. The species of the Middle Devonian genus 

 Ptericidhys^iiinnWaLV by the restoration of Traquair, attain a length 

 •of about 20 cm., while some species of the Upper Devonian genera 

 Bothrlohpis and Asteroh'pis are much larger |{. They were first 

 grouped as Antiarcha by Cope, when he supj)Osed he saw some 

 relationship between them and the Ascidians. 



* C. WiniRii, Bull. Geol. Tnst. Upjisala, vol. xvi. (1918), p. 94. 



t C. Wiman, loc. rit. 1918, p. 86, with text-fig. 



+ R. H. Traquair, Trans. Roy. 8oc. Ediub. vol. xxxix. (1899), p. 8.'54, pi. 4. 

 figs. 6-12, text-fig. 2 ; also loc. c'U. vol. xl. (190.')), p. 883, pi. 2. figs. 9, 10, 

 pi. 3, text-fig. 3. 



§ C. D. Waleott, Bull. Geol. Soc. America, vol. iii. (1892), p. 166, pi. 3. 

 figs. 6-14, pi. 4. figs. 1-4. C. R. Eastman, Proc. U.S. Nat. Mue. vol. Iii. 

 (1917), p. 2;i8. pi. 12. figs. .5, 6. 



II R. H. Traquair, ' A Monograph of the Fishes of the Old Red Sandstone 

 of Britain,' pt. ii. (Palajont. Soc. 1894-1913). 



LINN. SOC. PROCEEDINGS. — SESSION 1919-1920. d 



