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2) Of the six species of this genus known from colder seas (Arctic and Antarctic) four*
are characterized by having these bristles broken off in older females.
These facts indicate that external, climatic factors have perhaps caused this peculiar
convergence.**
It does not seem impossible that in the case of species which are characterized by having
the natatory bristles of the second antenna unbroken throughout the whole life the females
are impregnated several times. The statement made by G. W. Müller, 1894, p. 174, according
to which Ph. (Ph.) interpuncta lays at least two lots of eggs, seems to me to support this.
Genus Philomedes W. Lilljeborg.
Cypridina (part.) autorum; e. g. W. Baird, 1850 a and Fr. Müller, 1870.
Asterope, S. FISCHER, 1855. Philomedes (-= <J) + Cypridina (= $ and juv.), W. LilljebORG,
1853. Philomedes (= d) + Bradycinetus (= $ and juv.), G. 0. Sars, 1865; G. S. Brady,
1868 b and 1871. Philomedes ( - d: ? and juv.) autorum; e. g. G. 0. Sars, 1869
and 1887; G. W. MÜLLER, 1894; G. S. BRADY and A. M. Norman, 1896. Philomedes (part.),
G. W. Müller, 1912.
With regard to the relation of Pleoschisma and Tetragonodon to this genus cf. above
the remark below the sub-family.
Diagnosis: — Cf. G. 0. SARS, 1887, p. 45 and G. S. BRADY and A. M. Norman, 1896, p. 653.
Description: — Shell: — This has marked sexual dimorphism.
Female: — The shape of the shell varies rather considerably. The rostral in cisur
is of a somewhat varying type, but is most frequently deep and narrow. The sculpture of the
surface seems in most cases to be very weakly or even not at all developed ; in some cases,
on the other hånd, it is very powerful. Seen from inside: The part of the shell between
the list and the posterior margin of the shell is flattened and not curved in like a siphon, so that
the two valves are near each other at this part when the shell is closed ; only in exceptional cases
is there any indication of a siphon: (cf. the description of Ph. (Sderoconcha) Appellöf i).
Medial bristles: These had in the species investigated by me almost exactly the same type:
On the rostrum there was a row running at some distance from and almost parallel to the ventral
part of the anterior margin of the rostrum sometimes continuing a little way along the anterior
margin of the rostral incisur. Most of these bristles were rather long and finely feathered;
* It may lie poinled out thaï Ph. (Ph.) orbicularis, a species from the Antarctic, described by G. S. Brady,
vvhich is certainly very closely related to Ph. (Ph.) rotunda, has, according to this author’s dra wing, 1907, pl. I, fig. 8.
long unbroken natatory bristles on the second antenna; there is, however, no information as to whether the female from
which the antenna that is reproduced was taken had eggs in the brood chamber or if it was still in the plankton stage.
** See U. IIi’ssi: and ]■’. Doflf.in. ..Ti er bau und Tierleben“, 1914, 11, p. 876.
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