1316 THE VOYAGE OF H.M.S. CHALLENGER. 



Genus 582. Pterocorys,^ R&eckel, 1881, Prodromus, p. 435. 



Definition. — T h e o p i 1 i d a (vel Tricyrtida triradiata aperta) with three simple, 

 free lateral wings arising from the sides of the thorax. 



The genus Pterocorys, comprising numerous and some very common species, may 

 be one of the oldest and most primitive Theopdida, and is perhaps the common 

 ancestral form of all Tricyrtida. It exhibits three free lateral wings, arising from the 

 sides of the thorax, like Lithomelissa among the Dicyrtida, and may be derived directly 

 from this genus by development of an abdomen. The latter is commonly cylindrical or 

 ovate, with a wide open mouth, whilst the thorax is either campanulate or three- sided 

 and pyramidal. 



Subgenus 1. Pterocyrtidium, Biitschli, 1882, Zeitschr. f. wiss. Zool., vol. xxxvi. 



p. 531. 



Definition. — Horn of the cephalis simple. Abdomen not prolonged into a narrow 

 tube. 



1. Pterocorys campanula, n. sp. (PI. 71, fig. 3). 



Shell campanulate, with two distinct strictures. Length of the three joints = 2:3:2, 

 breadth = 2:4:6. Cephalis subovate, with a pyramidal horn of the same length. Thorax sub- 

 conical, in the middle zone with three conical, horizontal wings, shorter than the cephalis, from 

 which three thin ribs are decurrent to the mouth. Abdomen short and much dilated, with wide, 

 truncate mouth. Pores nearly equal, small, subregular, circular. 



Dimensions. — Length of the three joints, a 0'04, b 0-06, c 004 ; breadth, a 0-03, b 0-08, c 0-12. 



Habitat. — Central Pacific, Stations 266 to 274, surface. 



2. Pterocorys carinata, Haeckel. 



Eucyrtidium, carinatum, Haeckel, 1862, Monogr. d. Eadiol., p. 322, Taf. vii. figs. 4-7. 



Shell subovate, with two distinct strictures. Length of the three joints = 1:2:3, 

 breadth = 1:3:4. Cephalis ovate, with a pyramidal horn of twice the length. Thorax cam- 

 panulate, with three decurrent crests, from which in the lower part three small conical wings arise, 

 sliorter than the cephalis. Abdomen barrel-shaped, inflated, with wide, truncate mouth. Pores 

 everywhere equal, small, regular, circular, quincuncial. 



Dimensions. — Length of the three joints, a 0-25, b 0-05, c 0-075 ; breadth, a 0-25, b 0-09, c 01. 



Habitat. — Cosmopolitan ; Mediterranean, Atlantic, Pacific ; many stations, surface. 



' PieTOcor2/s= Helmet with wings ; vri^ou, x.6^vi. 



