74 DR. T. ALCOCK ON THE 



Next comes the order Scutibranchiata, including the 

 Trochuses_, Earshells^ Fissurellse, and some others. The 

 ribbon in these is comparatively short and broad ; and the 

 teeth in each tranverse series are generally very numerous, 

 and of three distinct kinds : in the first place, a median set, 

 consisting of a central tooth, with four or five similar ones 

 on each side of it, all having a lamellar form, with their body 

 laid flat on the supporting membrane, the recurved points 

 only projecting ; secondly, one or more large, strong, den- 

 tated teeth on each side, standing up boldly and overarching 

 the median set ; and thirdly, a numerous series of long, 

 slender, brush-like teeth, flanking the overarching ones on 

 each side, and generally forming a full fringe down the 

 borders of the ribbon. These particulars may be seen in 

 PI. V. The specimens by which I shall illustrate this type 

 of teeth are tongues of three species of Trochns, which, by 

 comparison, will give good illustrations of specific differences, 

 the tongue of an Earshell from California, of Fissurella ni" 

 gropunctata from Nicaragua, and of Glyphis inaqualis, an 

 animal which, from the characters of the teeth, must evi- 

 dently be quite distinct from Fissurella, although the shell 

 is similar. 



The third and last order I have to speak of is that of 

 the Pectinibranchiata, which includes a great majority of 

 all the marine spiral univalves, these being linked together 

 by a common type of general structure ; but still, as might 

 be expected in so large a number of forms, they present 

 strongly marked diff'erences in detail, by which the order 

 is subdivided into minor groups. Evidences of the exist- 

 ence of these distinct groups are at once seen on looking 

 over even a small series of their lingual ribbons ; and in 

 PL VI. a few of these varieties in the character of the 

 teeth are represented. One very marked difference, as 

 you will at once notice, is that, in some sets, the teeth are 

 in transverse series of three, while in others there are seven 



