INVERTEBEATE ANIMALS. 27 



Order PROSOBRANCHIATA. 7rpci(rov, in front ; 

 /Spayp^ja, gills. A very extensive order, comprising most 

 of the univalve Sea-shells best known in collections. 



Sub-order SCUTIBRANCHIATA. scutum, a 

 shield. Animals chiefly littoral in their habits, living 

 on rocks or sea- weeds near the shore. 



Section EDRIOPHTHALMA. 'i^pu, a seat; 

 6'^SaX[x,os, the eye. Eyes sessile. Shell, in the adult, 

 conical, not spiral. 

 Group 77.— Family CHITONID^. x«twv, a coat of mail. 

 Estimated number of species : recent, 255 ; British, 11 ; 

 fossil, 30, from the Devonian upwards. 



IT Examples of Lophyrus, Tonicia, Lorica, Crypto- 

 plax = Chitonellus, Mopalia, &c. 

 Upper Gompartmf.nt. 

 Plates of the shell of Leptochiton, separated to 

 illustrate the homology of the last plate with 

 the entire shell of Patella. 

 Group 78, — Family PATELLID^ and allies, patella, a dish; 

 Limpets. Estimated number of species: recent, 142; 

 British, 7 ; fossil, 100 ; from the Silurian upwards. 



^ Examples of Tectura, Scutellina, Gadinia, 

 Nacella, &c. 



Group 79.— Families FISSURELLID^ & DENTALIDiE. 

 Duck's-bill Limpets, Key-hole Limpets, Tooth-shells. 

 Estimated number of species : recent, 141 ; British, 7 ; 

 fossil, 144. Devonian, &c. 



The Dentalia have been regarded as Pteropods ; 

 also as FissurellcB, with highly conical and attenu- 

 ated shells. 



IT Examples of Antalis, Scutus = Parmophorus, 

 Emarginula, Cemoria, Lucapina, Tugali, Cra- 

 nopsis, Macrochisma, dtc. 



