On the Animals and Teeth of some Gaste^'opodoics Mollusca, 101 



of one and a half or two rapidly enlarging subconvolute whorls ; 

 aperture ovate, rather irregular, slightly dilated on the right side ; 

 cavity simple ; muscular scar subannular, with an angular inflection 

 rather behind the middle of the right side, the form of the scar is varia- 

 ble, sometimes square, broad all round ; in the larger, more developed 

 specimens the scar is rather horse-shoe shaped, being somewhat dilated 

 at the front part of each side, and the front portion over the back of 

 the head is narrow, linear, and transverse. 



The genus was first established by Rafinesque in 1814 ; Blainville, 

 who only knew it from Rafinesque' s imperfect descriptions, referred 

 it to the Patelloida, but Menke, Philippi and Cantraine properly con- 

 sidered it allied to Pleurobranchus, and especially Umbrella, and very 

 lately Dr. Loven stated that it was allied to Turbonella (Index Moll. 

 Scand. 1 9). The examination of the teeth proves it to belong to the 

 typical Pleurobranchiatay and the form and position of the gill shows 

 its affinity with the genera Pleurobranchus and XJmbrella ; indeed it 

 chiefly differs from the former genus in having an external conic 

 patelloid shell, and from the latter in the head being produced and 

 the mouth not sunken in a deep anterior pit. 



In the British Museum there are two species of this genus. 



1. T. punctulata, Rafin., T. Rajinesquii, Philippi, T. citrina, 

 Joannis, Guerin, Mag. Zool. i. t. 36. 



Shell thin, whitish ; periostraca hard, opake, with dark brown 

 rays. Mediterranean. 



2. T. atlantica= Umbrella mediterranea ? MacAndrew, Ann. 

 Nat. Hist. 



Shell solid, bright yellow ; periostraca — ? N. Atlantic, Madeira. 



Fam. IlMBRELLADiE. 



Umbrella mediterranea. 



The nucleus of this genus is very like that of Tylodina, subglobose, 

 polished, sinistral, of one and a half or almost two subcylindrical, ra- 

 pidly enlarging whorls ; the adult shell is irregular in the outline 

 and rather expanded on the hinder part of the right side, over the 

 gills ; the muscular scar is annular, continued, and of nearly uniform 

 breadth, but slightly interrupted in various parts. The chief dif- 

 ference between the shell of Tylodina and Umbrella is, that the shell 

 of the former is more elevated, very thin, covered with a hard, rather 

 paleaceous periostraca, and the muscular scar is furnished with an 

 angular inflation on the hinder parts of the right side ; a sinistral 

 nucleus is found on several others ; shells as in the genera of Pyra- 

 midellid<e. 



Fam. PROSERPINIDiE. 



Proserpina. 



Respiratory cavity open ; mantle free from the back of the neck, 

 with a double edge, the outer one rather reflexed ; foot moderate. 



