LECTURES ON MOLLUSCA. 83 



tection to particular organs of the body. The gills are not lodged in 

 a special neck-cavity, but are behind the heart. The sexes are united 

 in each individual. In the young state, they are exactly like the fry 

 of the prosobranchs ; each being inclosed in an operculatel spiral shell, 

 and furnished with pins and cilia. They are all inhabitants of the 

 sea. They are formed on two distinct types ; those in which the gills 

 are at the side, more or less covered by the mantle, and often protected 

 by a shell ; and those in which the gills are exposed, and entirely des- 

 titute of shell. They live principally on animal matter. 



ORDER I. TECTIBRANCHIATA. (Crawlers with sheltered gills.) 

 Family TORNATELLID^;. 



The animals of this tribe are as yet but little known. They are 

 arranged by Dr. Gray between Scalaria and Cerithiopsis, on the sup- 

 position that the gills are comb-like and the animal unisexual. It is 

 curious how large a proportion of existing observations on mollusks 

 need verification by those who have honest, well-trained eyes. Just 

 as the infant's eye has to be trained to distinguish forms and distances, 

 vso it requires practice before we know how to see truly an object that 

 lies before us. During the educational process it is often very easy to 

 see what we wish or expect to see. The shells of this tribe are nearly 

 allied both to the Pyramidellids and the Auriculids; and some aberrant 

 forms show relations both to Ovula and Dolium. As the living forms 

 are confined to a very few species, it is scarcely to be expected that we 

 should be able rightly to assign the positions of the various fossil 

 groups. These are found in great numbers, beginning with the coal 

 strata, becoming very plentiful in the oolites, and culminating in the 

 cretaceous age. The ordinary sculpture of the tribe is in spiral lines 

 or rows of dots. They differ from all the other Opisthobranchs in 

 having a very thin operculum, with broad, thin flaps, so as completely 

 to cover the mouth. The animal is quite retractile into the shell, and 

 has the general aspect of an Auricula, with its short, flat, triangular 

 tentacles and the eyes at their front. The teeth however are widely 

 different. Instead of the thousand tessellated teeth of the snails, there 

 are simply two rows of sickles arranged as an arrow head on the nar- 

 row, broad tongue. They live in . rather deep water, and are by no 

 means common in collections. The tentacles are used rather as a veil 

 than as feelers, being laid over the front of the shell in walking. The 

 gills are at the side, cloaked over by the mantle. 



The shells of Tornatella proper are thin, with onefold on the pillar. 

 Those of Buccinulus are stout, with two folds. (Monoptygma may prove 

 to be an elongated Tornatella, with a single, slanting fold.) All the 

 remaining genera are fossil. Acteonina is like a Monoptygma without 

 plait. The oolitic Cylindrites have the folds twisted outwards. The 

 chalk Acteonella is like a cone-shell with plaited pillar, but without 

 breathing notch. Cinulia has a globular shell, with many-plaited 

 pillar, and toothed outer-lip. Globiconcha has a similar shell without 

 the plaits. Varigera resembles it, with varices like Scarabus. In the 

 Portuguese Tylostoma, varices are formed thickened inside as in Cassis. 



