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



Genus 375. Phatnaspis,^ Haeckel, 1881, Prodromus, p. 468. 



Definition. — Belonaspida with eighty to two thousand or more parmal pores 

 (four to one hundred or more on each plate), without by-spines on the surface. 



The genus Phatnaspis corresponds to the spherical Coscinaspis among the Dorata- 

 spida, and differs from the other Belonaspida in the great number of the parmal pores ; 

 whilst the four other preceding genera exhibit only two opposite aspinal pores in the 

 centre of each plate, in this there are constantly numerous coronal pores in addition to 

 these, and the plates always possess the characteristic form of a wainscotted or panelled 

 work, with quadrangular meshes. The number of these parmal pores amounts in each 

 plate to from ten to twenty, often one hundred and twenty or more ; therefore the 

 number of parmal pores in the whole shell amounts to two thousand or more. Some- 

 times the pores are circular, but in this case too they are surrounded by quadranonlar 

 frames. The quadrangles are sometimes t[uite regular squares, sometimes more or less 

 irregular. The thin and fragile bars between the c[uadrangular pores form in each plate 

 two peculiar systems of parallel crests, which cross at right angles. Commonly the parallel 

 crests of one system (parallel to the major diameter of the compressed radial spines) 

 are equidistant, and pierce from one edge of the plate to the opposite, whilst the 

 parallel crests of the other system (parallel to the minor diameter of the .spines) are 

 interrupted and at diflferent distances (PI. 136, fig. 9) ; but in other sjDecies both 

 crossed systems are quite regular. In each plate there is one primary diagonal rib 

 (often stronger than the parallel crests) which connects the two opposite corners of the 

 rhomboidal plate. We can distinguish in this genus three diflerent subgenei'a : A. in 

 Phatnasparium the primary diagonal rib arises from both flat sides of the compressed 

 sword-like, radial spines ; B. in Phatnasplenium from both sharp edges of them ; C. in 

 PhatiHtsjndium two crossed diagonal ribs arise from four edges of the spines (combina- 

 tion of A and B). Therefore in the first subgenus (A) two primary aspinal pores are 

 placed opposite on the sharp edges of the spines, but in the second (B) inversely on theii' 

 flat sides ; in the third (C) there are apparently four primary aspinal pores, which are 

 probably derived from B or A by division of the two pores. 



Subgenus 1. Phatnasparium, Haeckel. 



Definition. — In the centre of each rhomboidal plate two primary aspinal pores, 

 opposite on both edges of the compressed spines, from both flat sides of w^hicli the 

 primary diagonal crest arises. 



1 P/uifnospis = Panel-shield or fretwork ; ©aTm. iuir/?. 



