16 CRETACEOUS GASTROPODA 
which had been observed, two families were proposed: (a) S7vromprpZ, with Ptero- 
cera, Strombus and Rostellaria as the chief genera; and (b) APporrzarps, with 
Aporrhais and Struthiolaria (and probably Priamus), to which Gray adds Tricho- 
tropis and others (possibly better separated into a distinct group). Now viewing the 
similarities and the differences of the two families, we see them to be both Rosrrirpra 
in the sense of Gray* (Guide to Mollusca, 1857, page 64), that is, 
Siphonostomata with an annulated rostrum, subulate tentacles with the eyes on 
their outer side, and the teeth in seven series: three on either side being respectively of 
similar form. The mantle has its edges more or less expanded and lobed; the gilis 
are pectinate; the operculum is annular, ovately elongated, rather thin and horny : 
the shell is fusiform, turrited or more ovate, the whorls internally compressed, narrow, 
and the outer lip always somewhat expanded in adult age. 
The principal differences recorded in the anatomy of the animals, exist in the 
locomotive organs and the tentacles. In Strombus, Pterocera, Rostellaria (considering 
these genera in the old sense of Lamarck) the foot is divided into two parts, the 
posterior being more or less prolonged, slender, and bearing on its termination a 
claw-like operculum. The eyes are on prolonged peduncles, which have the tentacles 
on the internal side, that is to say, morphologically, the eye-peduncles and the tentacles 
seem to be grown together for a longer or shorter distance. The tentacles are 
often longer than the peduncles in Rostellaria, and nearly of equal size with them 
in Plerocera. In Terebellum, the forepart of the divided foot is very small and the 
hinder part very prominent and thick. The tentacles are usually said to be wanting, 
and the eyes to be on the ends of very long peduncles. Gray questions the first point, 
and it is, we think, very probable, that further researches will prove, that the ten- 
tacles are united, or grown together with the eye-pedicles throughout their entire 
length. This ought, at least, to be anticipated from a morphological point of view. 
Aporrhais (Chenopus, Phill.) has the foot not divided, but somewhat com- 
pressed, and, although truncate in front, still somewhat produced; the eyes are 
sessile on very short peduncles, which are united with the very long subulate tentacles 
at the base only. Gray (teste Clark) says (Guide to Mollusca, page 75) the “4. pes- 
“ nelicani creeps slowly ; but the organs do not appear adapted for progressive move- 
«ment. It is shy, and whether the shell is placed with the aperture upwards or 
« downwards, it does not usually commence creeping by pushing out the foot anteriorly 
“like other Gastropoda, but often twists the long neck and foot to the caudal extremity, 
“ and there fixing it, with a sudden spring effects the turning of the shell.” It is 
obvious, that this kind of movement is quite similar to that of Plerocera or Strombus, 
except that the divisions of foot and caudal extremity are not so evidently separated, 
as in those two groups of shells. 
The foot of Struthiolaria is thick, small and oblong, more adapted for fastening 
than for creeping. The tentacles are of moderate length, and the eyes are on small 
bulgings outside near the base. 
We see thus, that in the different alterations in the form of the foot and the eve- 
pedicles there is a transition and connection, rather than a strict separation. The 
* This author proposes for the Alata, excluding Aporrhais and others, but including Onustus and Phorus, the 
name of Lepropona, on account of the divided foot and the manner of moving about. 
