Page:Scientific results HMS Challenger vol 18 part 1.djvu/1076

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Definition.— 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 Dorataspida, 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. Sometimes the pores are circular, but in this case too they are surrounded by quadrangular frames. The quadrangles are sometimes quite regular squares, sometimes more or less irregular. The thin and fragile bars between the quadrangular 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 different distances (Pl. 136, fig. 9); but in other species 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 different subgenera: 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 Phatnaspidium two crossed diagonal ribs arise from four edges of the spines (combination 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 their 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.

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 which the primary diagonal crest arises.