206 LECTURES ON 



one species were long ago ascertained by Adanson, the very accurate 

 though eccentric naturalist of the west African coast. Since that 

 time the animals have been so far investigated that various genera 

 have been established by Dr. T. E. Gray and others. The typical 

 Vermetids begin life free, and so continue for some time, making a 

 delicate spiral shell exactly like Turritella. They then begin to tire 

 of their freedom, and long for some protecting support, They sud- 

 denly give up their beautiful spiral shape, and twist themselves any- 

 how in search of a secure foundation. Having moored themselves to 

 it for life they writhe in shelly contortions, crystallized (so to speak) 

 as soon as formed, during the remainder of their sedentary existence. 

 Of this tribe, the Ivory Worm-shell, (V. eburneus,) furnished by our 

 Thorn-Oysters, is perhaps the most beautiful species. 



But the other Vermetids for the most part only show their con- 

 nexion with spiral shells in the nuculear portion. These shells, in 

 the Indo-Pacific ocean, generally have a deeply concave operculum,* 

 with only a loose trace of spiral development. Of these, many 

 specimens were brought home by the Exploring Expedition, unfortu- 

 nately without the animal. But it was found that though the Gulf 

 shells could hardly be distinguished from the Australian, their oper- 

 cula most resembled those of Turritella. This was the more remark- 

 able as the Turritelloid worm-shells have a very different operculum. 

 The new group was named Aletes, the "Wanderer." It appears, how- 

 ever, not to wander from the west coast of America: those found 

 from the Mediterranean to the Pacific islands (as far as known) all 

 belonging to the concave type. Wandering among the Wanderers 

 are some equally large and equally highly-colored shells with an oper- 

 culum formed on a still more intricate pattern. From the horny plate 

 rise up two long processes branching exactly like stag's horns. They 

 are made, however, by a headless annelid, and are not so much worm- 

 shells as shell-worms. The Annelids have generally an earthenware 

 texture; while the Vermetids are porcellanous. But on breaking 

 across some specimens of a very small species, closely though irregu- 

 larly twisted, I was surprised to find a structure which had not before 

 been described in any recent spiral shell. An extremely thin lamina, 

 like the deck of the slipper-limpets, proceeded from one wall of the 

 shell, doubled itself over at a right angle, and met a similar lamina 

 proceeding from below, so as almost to divide the body of the animal 

 into two parts one within the other. 



Petaloconchus macrophragma.— Section across the shell at three periods of growth : a, one lamina first com- 

 mencing, or ending; 6, upper lamina prolonged, lower commencing; c, upper lamina doubled over to meet 

 the lower, while a rudimentary one appears between. 



* The operculum is a horny or shelly appendage to the end of the foot, drawn in last 

 •when the animal retires into its shell, and thus protecting it. It may be called in com- 

 mon language the trap-door or toe-nail. 



