LINXEAN SOCIETY OK LOXiJON. 2? 



AXASPIUA. 



The most isolated and least-known order of the Ostracodernis 

 is that ol: the Anas])ida, which are either fusiform free-swimmers 

 or elongated and almost eel-shaped. They ai'e usuallj^ only from 

 10 to 15 cm. in length, and with one exception they are known 

 only from the Do wnt on ian Passage Beds at the top of the Silurian 

 System. They are prohably to he regarded as the latest survivors 

 of the. ancestral Ostracoderms. which were beginning to acquire 

 a hard dermal skeleton at the end of Silurian times. 



The Anaspida were first described from Ayrshire and Lanark- 

 shire by Traquair *, who recognised two genera, Birkcnia and 

 Lasankis. Birkenia is completely covered with scales, which are 

 fusiform and rather irregularly arranged on tlie head, but deep 

 and narrow and disposed in oblique lines inclined forwards and 

 downwards on the trunk. A single row of enlarged scutes 

 extends along the lower border of the trunk ; and small scales 

 take the place of fin-rays on the single dorsal fin and on the 

 lower lobe of the distinctly heterocercal tail. Low on the side of 

 the head there is the orbit, surrounded by large plates resembling 

 the circumorbital plates of the AcanLhodian fishes ; behind the 

 head aii oblicpie line of pores may be interpreted as gill-openings; 

 while a douhle scute at about the mitidle of the ventral series 

 ])robably marks the cloaca. Otherwise, there are no indications 

 of the internal parts of the animal. Lasanius occurs as a mere 

 stain on the rock, bounded below by the single series of ventral 

 scutes, and partially armoured only in the foremost part of the 

 trunk by a few oblique rows of scales, which are fused into rods 

 showing a triangular expansion only at the point where they are 

 crossed by the lateral line. The forked heterocercal tail is dis- 

 tinct, but I have never seen any ordinary fin-rays in its lower 

 lotie. The eye is marked by a dark stain ; and slight dermal 

 cnlcifications seem to indicate the position of the row of supposed 

 branchial openings corresponding with those of Birkenia. 



A fraguient, either of Birkenia or of a relatetl genus, has been 

 found in rocks of the same age in New Brunswick, Canada T, and 

 three other Anaspida are now known from the Downtonian 

 formation of southern Norway j. The latter have not yet been 

 fully described, but one of them (Pterolejns nitidus) is specially 

 interesting because its dorsal fin is armed with an anterior spine. 

 Detached fin-spines [Oncltvs) of Silurian age have generally been 

 regarded as referable to Elasniobranch fishes, but this new dis- 

 covery shows that they may belong to vertebrates of much lower 

 grade, 



» R. H. Traquair, Trans. Roy. Soe. Ediub. vol. xxxix. (189!)), pp. 837-843, 

 pi. 5 ; also loc. cit. vol. xl. (I'JOij), pp. S8()-7, pi. •-'. figs. 4-8. 



t CtnKipfciiron ncrepii^oi^i', tr. F. I\Iattlie\v, Trans. Hoy. Soc. Canada, ser. 3, 

 vol. i. (1!H)7}, sect. 4, p. 7, pi. 1. 



\ J. Kiasr, Skritt. Videusk.-Seltk. Kri.'^tiania, Mat.-natiuv. XL 1911, no. 7, 

 pp. 17-19. 



