unwound or loosely coiled shell, and in development of ciliary feeding, accompanied by a crystalline style in the gut. In structural details, however, there is no very close correspondence, either in the foot and pallial region, or in the alimentary canal. The first group includes “Vermetus” (s.str.), Serpulorbis, Aletes, Spiroglyphus, and Bivonia, and probably also the New Zealand genus Novastoa, of which a detailed account is in the course of preparation. In the second group may be associated together the smaller operculate vermetids of the genera Stephopoma, Siliquaria and Pyxipoma. The following summary of structural differences marks a fundamental line of division in the Vermetidae.

The origin in the two groups of the sessile habit and the uncoiled shell is possibly to be sought independently among separate stocks of free-moving style bearing mesogastropods. On the one hand the main evolutionary trend has been from ciliary feeding towards mucous trap feeding, with the accompanying enlargement of the pedal gland and tentacles and reduction of the gill. In the second group, as typified by Stephopoma, ciliary feeding adaptations are more perfect, and evolution seems to lead finally to the sweeping net mode of food capture. Neither pedal gland nor pedal tentacles are present.

The comparative table also brings out the close resemblance that exists between Stephopoma and the siliquariids (as typified by Pyxipoma). Thiele (1931) conservatively divided the Vermetidae into two large “generic” sections—Vermetus, comprising nine sub-generic groups, including among them Stephopoma, each better recognised as genera—and Tenagodus (equals Siliquaria s. lat.) distinguished by the longitudinally fissured shell. This shell distinction is now seen

to be of less fundamental importance than has been hitherto supposed. Important similarities have been pointed out between Stephopoma and the siliquariids in the head, foot, pallial cavity, and the digestive and reproductive systems, indicating that these vermetids should be placed together in a single natural group. The principal diagnostic feature is the embryo shell, which is exactly alike in shape in Stephopoma and Pyxipoma, consisting of 1½ whorls coiled in an almost plane spiral, of nautiloid or limacinid shape. The mouth is wide, circular and trumpet shaped, with the peristome projecting freely around the inception of the adult shell. The embryonic operculum is in both cases a simple circular disc. The adult shell is coiled in a regularly increasing corkscrew spiral, generally embedded in or loosely attached to the substratum, never firmly cemented as in Serpulorbis, Aletes or Spiroglyphus. The radula (Figs. 22, 23) is at once distinguishable from that of previously described vermetids. The operculum is formed of a spirally coiled, setose band, a flat spiral with complex setae in Stephopoma, and in Pyxipoma elevated to form a tall dome, with the margin simply setose. The full significance of the shell fissure in the siliquariids remains to be worked out with observations on living material: the slit would seem to have arisen as a more efficient adaptation for the removal of rejected waste material from the mantle cavity.

As regards the Stephopoma-Siliquaria section of the vermetids, the closest relationships of these genera would appear to be with the Turritellidae. While allowing for the convergent adaptations that are especially likely to exist in specialised groups of ciliary feeders, we may recognize in the account by Graham (1939) of Turritella communis several important resemblances to Stephopoma roseum as described above. In Turritella and Stephopoma, the pallial organs are similar, in the endostyle, hypobranchial gland and food groove. In both cases the gill filaments are narrow and linear, in adaptation to ciliary feeding; in Stephopoma, however, they are especially slender and cirriform, in relation to the sweeping fringe mode of feeding. The mantle margin in Stephopoma is fringed with simple papilliform tentacles which are present also in Turritella, which has as well an additional series of pinnate guarding tentacles, protecting the entrance to the pallial cavity. These are unrepresented in Stephopoma, being perhaps replaced functionally to some extent by the long opercular setae. Turritella retains a much more generalised structure of the foot being free-moving and using the sole for creeping in the normal fashion; it is significant that the operculum is fringed with simple setae, perhaps foreshadowing the condition found in Stephopoma and Siliquaria. The exhalent siphonal appendages described by Graham in Turritella are represented by the triangular siphonal tubercle on the right side of the foot in Stephopoma. Further, in both groups the snout is short, the jaws reduced, and the food bolus is seized by the radula, while the structure of the pharynx, salivary glands and oesophagus is almost identical. The stomach of Turritella and Stephopoma are extremely similar, in the shape and proportions of the style caeum, the relations of the sorting area, and the presence of a crescentic fold, with a deep groove from which opens the single digestive diverticulum. In both forms the intestine originates as a narrow groove between paired typhlosoles, and the digestive gland shows detailed