380 DITRYPA ARIETINA. 
1835. Serpula, Sars. Beskrivelser, p. 52. 1842. Ditrupa, Agassiz. Nomenclat., p. 3, Vermes. 
1844. Cadulus ?, Philippi. Enumeratio, 1, p. 208, t. xxvii, fig. 21. 1844. Ditrypa, Philippi. 
Arch. f. Naturges. 1847. Gadila, Gray. Proc. Zool. Soc., p. 159, n. 280. 1848. Ditrupa, 
Brown. Index Pal., 1, p. 433. 1851. Serpula and Placostegus, Grube. Fam. Annel., pp. 91 and 142. 
Collar diaphanous, well-developed, edge laciniated, fissured in the mid-dorsal line 
but continuous ventrally. Branchize in two fans of considerable length, the filaments ending 
in short processes. Pinne rather long. Pedicle of the operculum springs from the left 
branchial fan, and supports a long vase with the calcareous plate on the top. Anterior region 
of the body of six bristled segments. Tail with two rounded papille at the vent. Alar 
membrane rudimentary, but with a bifid ventral flap posteriorly. Capillary anterior bristles 
with narrow wings on the tapered tips. Posterior bristles delicate, confined to the caudal 
region, and of similar structure. Anterior hooks avicular, with numerous small teeth on 
the anterior edge. Posterior hooks smaller, but agree in structure. Tube shaped like a 
slender tusk of an elephant, obliquely thinned at the aperture. Surface smooth or marked 
by faint lines of growth. | 
1. Dirrypa arietTina, O. F. Miller, 1776. Plate CX XII, fig. 5—body, fig. 6—branchia, 
figs. 6a and 6b—tubes ; Plate CX XXII, fig. 1—bristles and hooks. 
Specific Characters.—Collar diaphanous, deep and thin, with the edges usually laciniated, 
fissured in the mid-dorsal line, but thereafter continuous from side to side. No alar membrane 
is visible dorsally. Branchiee of considerable length, in two fans, the filaments tapering 
from base to apex, and ending in a short process, which is slightly longer than the adjoming 
pinne, though it is twice as thick, whilst its surface has cilia. The pinne are long and so 
arranged distally that they form a nearly even series at the tip. In life they are banded with 
red. Pedicle of the operculum springs from the left branchial fan, is long, slender, flattened 
below, and rounded distally, where it joins the long and shapely vase, with the calcareous, 
flattened yellowish-green plate on the top. Body widest in front, though it is usually com- 
pressed into a cylindrical form by the tube, tapered a little posteriorly, and ending in a conical 
and somewhat flattened tail, with two rounded papille (cirri) posteriorly. Anterior region 
of six bristled segments ; alar membrane rudimentary, but with a bifid ventral flap posteriorly ; 
posterior region of many narrow segments. Anterior bristles, directed upward and backward, 
enlarged a little above the insertion, remain cylindrical to the commencement of the tip, 
which tapers to a delicate point and has narrow wings. Posterior bristles confined to segments 
near the tail; simple, delicate and tapered. Anterior hooks avicular, with numerous small 
teeth on the anterior edge, and a prominent but not sharp main fang, with only a shallow 
notch below, and a rudiment of a prow behind it. Posterior outline with only a trace of an 
incurvation. Body of hook striated. Posterior hooks agree im structure, though fewer and 
smaller. Tube shaped like a slender tusk of an elephant, with the smooth and circular 
anterior aperture encircled by an obliquely thinned wall. Outer and thicker parts of wall 
of tube vitreous, inner layer opaque white (porcellanous). 
Philippi (1844) did not include this genus under the Serpulids. 
