1326 THE VOYAGE OF H.M.S. CHALLENGED. 



roundish, of different sizes. On the peculiar symmetry of this remarkable form, compare the 

 detailed description in my Monograph. 



Dimensions. Length of the three joints, a 0'02, J 0'06, c 0'08 ; breadth, a 0'02, b 01, c 012. 



Habitat. Mediterranean, Messina, surface. 



Gemi3 586. Pteropilium, 1 Haeckel, 1881, Prodromus, p. 435. 



Definition. Theopilida (vel Tricyrtida triradiata aperta) with three latticed 

 free lateral wings on the sides of the thorax, prolonged into the horn of the cephalis in 

 the form of three vertical, free cephalic wings. 



The genus Pteropilium differs from the preceding closely allied Dictyoceras in the 

 development of three free, vertical, latticed cephalic wings, expanded between the apical 

 horn of the cephalis and the three wings of the thorax ; they may be direct prolongations 

 of the latter. Pteropilium, however, may also be derived directly from the Dicyrtida 

 Callimitra or Clathrocorys by development of an abdomen. All these interesting 

 forms are closely allied to the Plectoidea. 



Subgenus 1. Clathropilium, Haeckel, 1881, Prodromus, p. 435. 



Definition. Thorax completely latticed, without three larger lateral holes between 

 the three wings. 



1. Pteropilium stratiotes, n. sp. (PI. 70, figs. 9, 10). 



Shell thin-walled, with two deep strictures. Length of the three joints = 1:5:4, 

 breadth = 2 : 6 : 9. Cephalis hemispherical, with a vertical, straight, prismatic horn of three 

 times the length. Thorax inflated, oblique, with three slender, nearly straight, widely divergent 

 ribs, beginning from the collar stricture and united in its centre with the centripetal prolongation 

 of the horn. The basal half of the three ribs is enclosed in the thorax wall, whilst the distal half 

 lies outside it, and is connected with its lower half only by two divergent, reticulated wings. The 

 upper edge of the ribs is connected with the occipital horn by a vertical, loosely reticulated, 

 triangular wing. Abdomen half as long as the thorax, also of irregular and oblique form, with 

 knee-shaped outline and a wide, truncate mouth. Network with irregular polygonal, for the most 

 part quadrangular, meshes, separated by thin bars and forming irregular transverse rows. Some 

 stronger, vein-like ribs ramify in the wall, diverging towards the mouth. 



Dimensions. Length of the three joints, a 0'03, b 01, c 0'08 ; breadth, a 0'04, b 012, c 018. 



Habitat. Central Pacific, Station 271, surface. 



1 Pteropilium = Small hat with wings ; 



