GEODIA BREVIANA. 157 



or obliquely. They traverse the thin dermal layer and protrude freely beyond 

 its outer surface. The spicule-fur appears to have consisted of a high portion, 

 composed of the distal parts of the large amphioxes, anatriaenes, and perhaps 

 also mesoprotriaenes protruding several millimeters; and a low portion, forming 

 a sort of undergrowth, composed of the distal parts of the minute dermal am- 

 phioxes and anaclades, protruding only 100-200 /i. 



The microscleres are thick-rayed and perhaps also thin-rayed oxyasters, 

 large oxysphaer asters, small strongylosphaerasters, and sterrasters. The thick- 

 rayed oxyasters are numerous throughout the choanosome. The thin-rayed 

 oxyasters are rare and were found only in the centrifugal spicule-preparations ; 

 they may be foreign. The large oxysphaerasters occur chiefly in the walls of 

 the subcortical canal. The strongylosphaerasters form a single layer on the 

 outer surface and also occur scattered in the lower parts of the cortex. The 

 sterrasters occupy the sterraster-armour layer in dense masses. 



The large amphioxes (Plate 36, figs. 1-4) are slightly curved and uniformly 

 and gradually attenuated towards their pointed ends. They measure 1.8-3.7 

 mm. in length and 30-88 /t in thickness. Their thickness is by no means always 

 proportional to their length. In the specimen described by Lambe the ordinary 

 choanosomal amphioxes are 3-5 mm. by 50-70 f^ (Lambe, 1893, 2.77-3.81 mm. 

 by 80 li). 



In this specimen I have also found some large styles; these are shorter than 

 the amphioxes and up to 90 ju thick; they are not mentioned by Lambe. 



The minute dermal amphioxes (Plate 36, figs. 10-12) are simply curved 

 (Plate 36, fig. 10) or angularly bent (Plate 36, figs. 11, 12) and gradually attenu- 

 ated towards the abruptly pointed ends. Examination with high powers 

 shows that both ends of these spicules are usually somewhat drawn out to ex- 

 ceedingly sharp terminal spines. The angular bend is, when present, generally 

 not in the middle of the spicule, but considerably nearer to one end than to the 

 other. It amounts to about 25°, so that the angle which the two parts of the 

 spicule enclose is usually about 165°. The minute dermal amphioxes are 280- 

 450 [x long and 4.5-8.5 [i thick. The shorter ones, 280-365 ^t in length, are much 

 more numerous than the longer ones, 366-450 ji in length. In the specimen 

 described by Lambe these amphioxes are 340-440 ;« by 2-5 n (Lambe, 1893, 288 

 by 13 /«, according to his figure 8.5 /<)• 



The ortho- and plagio-triaenes (Plate 35, figs. 15-17) have a fairly straight 

 rhabdome, 1.8-3.2 mm. long and 90-130 /( thick at the cladome. Usually the 

 rhabdome is conical and sharp pointed, rarely rounded at the acladomal end. 



