hexaptera^ various authors. 



to 70 



20 — 25 



6—8 



3—4 



4—7 



magna Langerhans. 



to 40 



— 



7 — 9 



4 



- 3 



magna Grassi 



to 41 



20 



10—13 



4 



2—3 



tricuspidata Kent 



— 



20 



8 



3 



1 



tricuspidata Doncaster 



nearly 40 



— 



4-8 



- 3 



1—4 



Among the Siboga material were specimens which fitted sufficiently to each of these 

 formulae (except for the large number of jaws in magna Grassi). On adding to them specimens 

 of magna supplied from the Zoological Station at Naples, one felt fairly confident that one 

 had all the species before one. Good specimens were selected and compared, with the result 

 that no specific difference could be detected between them : in all ordinary characters such as 

 the .position and extension of the fins, the size and position of the abdominal ganglion, the 

 extension and shape of the corona, the proportion of tail to trunk, and so forth, they were 

 practically identical ; the vestibular ridges, and even the tips of the younger jaws, were of the 

 same character. I have no doubt that hexaptera d'Orbigny, magna Langerhans and tricuspidata 

 Kent, form one species. Whether magna Grassi is the same or not, is not so easy to say : some 

 of my specimens showed nine jaws with a rudimentary tenth, but ten was the outside number 

 observed. But as the specimens of "■magna' sent from Naples were undoubtedly hexapteran, 



44 



20 



7 



42 



19 



6 



40 



2 2 



7 



3 5 



3 6 



it is probable that Grassi had before him specimens of hexaptera with numerous jaws and 

 few teeth ; such occur in the table below. 



Since there is practically nothing in their original definitions to differentiate magna and 

 tricuspidata from one another and from hexaptera except the formulae for armature, it is 

 necessary to show the resemblance between specimens which conform more or less to the 

 formulae; this has been done on plate I. The outlines of the entire animal 1 ) (figs. 30, 34, 38) 

 are as alike as could be expected in three different specimens of the same species ; the two 

 coronae (figs. 35, 39) agree with each other and with that figured by Hertwig for hexaptera 

 (op. cit., pi. IV, fig. 21); those of the distal end of the latest (most anterior) jaw agree in 

 the characteristic outline first recorded by Krumbach (Uber die Greifhaken der Chatognathen. 

 Zoologische Jahrbiicher, Abtheilung Systematik, u. s. w., XVIII, p. 579, fig. P.); if the figures 

 of the anterior and posterior teeth do not at first sight agree equally well, the failure must be 

 attributed to the impossibility of getting them to lie at precisely similar angles, and to the 

 inability of the draughtsman to represent properly the consequent foreshortening. By focussing 

 it was apparent that they belonged to the same type, — a broad base from which rises an 

 unusually long and slender tooth. In such large, and therefore opaque, heads as most hexaptera 

 present, it is generally impossible to see the whole of the vestibular ridge under the microscope 



1 1 As typical specimens of magna were only noticed in the alcohol material, the outline for this "species" has been taken 

 from a Naples specimen. 



