1. The Radula

In radular characters, Aporrhais may be held to represent the basal type in the superfamily Strombacea, and an illustration is now provided of the radula of A. pes-pelicani (Text-fig. 1a) showing greater detail than the reproduction from Troschel (1856) figured by Morton (1950). The central tooth is broadly rounded anteriorly and tapers to a narrow flange behind. The central cusp of this tooth is short and broad, consisting of a single large denticle, flanked by two pairs of smaller ones, and with a series of 3 or 4 tiny cusps at either side, along the anterior margin of the tooth. The lateral teeth are triangular in shape and very broad, about three times the breadth of the central, with a broadly curved anterior margin, from which denticles are entirely lacking or very obsolete. The marginals are slender and narrow, strongly curved, and much longer in proportion to the rest of the radula than are the reduced marginals in the struthiolariids. They have no trace of separate cusps or denticles, the tips being smooth. It is probable that in A. pes-pelicani, the species of which it was most easy to obtain the animal, the radula is in some features specialised in comparison with that of other aporrhaids, as for example in the great breadth of the laterals and in the lack of denticulations on these and on the marginals.

Perissodonta mirabilis (Text-fig. 1b) agrees with P. georgiana (as illustrated by Morton (1950) and Powell (1951) in the aberrant feature of multiplication of the marginal teeth. The single specimen of P. mirabilis examined has four pairs of marginals; in P. georgiana, there may be four or sometimes five. In other features the radula is clearly intermediate between the aporrhaid and the modern struthiolariid condition. The separate denticles of the central tooth are already arranging themselves at either side of the median denticle into a single, large, triangular cusp, and the individual denticles tend to become reduced in size. The central tooth is rounded in front, narrowing behind, and thus still displays the aporrhaid shape, as does also the lateral tooth, though in Perissodonta the triangle of the lateral tooth is much less broad, scarcely if at all wider than the central tooth. Moreover, the lateral tooth is strongly cusped with a large antero-median denticle extending towards the midline, together with a row of about 12 smaller denticles along the anterior edge. The marginals are also finely cusped, resembling those of Struthiolaria (s. str). The size of the radula relative to the size of the animal is greatly reduced in all the Struthiolariidae as compared with the Aporrhaidae, as may be seen from the scales provided with the radulae shown in Text-fig. 1. (It should be remembered that the animal of Aporrhais pes-pelicani is of rather less than half the actual size of the two struthiolariid animals considered.) As ciliary feeders, the Struthiolariidae show a well-marked general trend towards reduction of the radula; the smaller denticles are indeed of such minute size that most of the evolution of the radula in this family has probably taken place well outside the field of adaptive play.

In Tylospira (Text-fig. 1c) the central tooth has assumed the modern struthiolariid shape, straight across the anterior edge, and broadly rounded or obtusely pointed behind. Its central cusp is of the more advanced, triangular form, finely serrate along its straight free edges. It is much elongated, so as to project backwards over the anterior margin of the succeeding tooth, and in this feature it most resembles Pelicaria. This likeness is shared by the lateral teeth, which are now rectangular plates, with a single antero-median cusp, triangular in shape,