175S THE VOYAGE OF H.M.S. CHALLENGEE. 



2. Cceloplegma tritonis, n. sp. (PI. 127, figs. 2—13). 



Shell-mantle one and a quarter times as long as broad, its frontal perimeter heptagonal, with 

 seven concave sides (fig. 2), its sagittal perimeter ovate, its zonal perimeter nearly square (fig. 3). 

 Odd nasal style with twelve to fourteen, the paired pectoral styles with three to four, lateral styles 

 with six to eight, and tergal styles with eight to ten pairs of branches. Coronets three times 

 forked, each with eight terminal branches, bearing a small spinulate knol). This remarkable 

 species, in external appearance very different from the preceding, is connected immediately with it 

 by numerous transitional varieties ; both species represent the opposite terminal poles of a long series 

 of " Darwinian metamori^hic forms." If only the two specimens, figured in PL 127, figs. 1 and 2, 

 were known, every one would distinguish them as two widely different species. But the careful 

 comparison of numerous intermediate forms demonstrates that there is no " missing link " in this 

 long and remarkable chain. The careful comparative study of these very variable and most highly 

 developed Ccelographida may be regarded as a strong argument for the theory of descent, and 

 explains the true " origin of species." 



Dimensicms. — Length of the shell 1'8 to 2'5, breadth 1-5 to 21. 



Ealitat. — North Atlantic, Gulf Stream, off the Fseroe Channel, in depths between 40 and 200 

 fathoms, John Murray. 



3. Cceloplegma tetradecastylum, n. sp. 



Shell-mantle one and a third times as long as broad, its frontal perimeter heptagonal, with 

 seven concave sides. Similar to the preceding species, differs from it mainly in the prolonged nasal 

 style, which bears sixteen to eighteen pairs of branches, and is about three times as long as each of 

 the six paired styles, each of which bears five to six pairs of branches. Coronets three times forked, 

 each with eight terminal branches, armed with scattered spines, and bearing a knob with three 

 recurved hooks. 



Dimensions. — Length of the shell 3'2, breadth 24. 



Hahiiat. — Tropical Atlantic, Stations 335 to 342, surface. 



4. Cosloplegma atlanticum, n. sp. 



Shell-mantle one and a half times as long as broad, its frontal perimeter ovate. Similar to 

 Cceloplegma murrayanum (PI. 127, fig. 1), differs from it in the different length of the styles ; the 

 nasal odd style (with eighteen to twenty pairs of branches) is about twice as long as the lateral 

 styles (with ten to twelve pairs), and three times as long as the pectoral and tergal styles (with six 

 to eight pairs). Coronets four times forked, each with sixteen terminal branches, armed with 

 recurved spines, and bearing a cross of four curved hooks. 



Dimensions. — Length of the shell 2-5, breadth 1-7. 



ffabitat.South Atlantic, Station 318, depth 2040 fathoms. 



