KEY TO THE FAMILIES. XIX 



7. Shell none. 



;. Grills exposed, arranged round the vent ... ... ... 53 



:. Gills exposed, arranged on each side ... ... ... 54 



ONCHIDIHXE. Marine, gills not exposed ... ... ... ... 54 



Class Lamellibranchiata. 



Shell composed of two valves. 



1. Pallial line sinuated. 



(<r.) Ligament none. 



PHOLADIM:. Hinge with accessory plates ... ... ... 58 



(b.) Ligament external. 



G-LYCIMEEIM:. Equivalve, thick, gaping ... ... ... 60 



TELLINID^E (part). Compressed, slightly gaping, thin ... ... 65 



;. Equivalve, three diverging cardinal teeth in each valve 69 



:. Elongate, thin, gaping at both ends ... ... 73 



(c.) Ligament internal. 



Small, thick, produced behind ... ... ... 60 



;. Pearly inside, cartilage with a free ossicle ... ... 61 



:. Trigonal, ligament in a deep triangular pit, two di- 

 verging cardinal teeth ... ... ... ... ... 62 



TELI/CNID^E (part). Thick, ligament in a deep triangular pit, one 



small cardinal tooth ... ... ... ... ... 65 



2. Pallial line simple. 



(.) Ligament external. 



CAEDIID.S;. Swollen, posterior end unlike the rest ... ... 73 



ISOCAEDIID^;. Kadiating nodulous ribs ... ... ... 74 



LTTCINID.E. Sub-orbicular, two cardinal teeth in each valve, and 



two laterals ... ... ... ... ... ... 74 



- UNGHTIINID^;. Sub-orbicular, two cardinal teeth in each valve, but 



no laterals ... ... ... ... ... ... 75 



SOLEMYADA Covered with a long skin ... ... ... 76 



(b.) Ligament internal. 



* Two adductor impressions. 



EEYCINID.E. Small, thin ; one or two cardinal teeth and laterals 75 

 CBASSATELLiD.ffi. Thick, sub-trigonal, two anterior lateral teeth 



in each valve ... ... ... ... ... ... 76 



MYTILID.&:. Umbones anterior, shell closed ... ... ... 77 



AvicuiiD-ffi. Umbones anterior, shell gaping ... ... 79 



ARCID.E. Hinge with many teeth in a line, not pearly inside ... 79 



NucrLLDJE. Hinge with many teeth in a line, pearly inside ... 80 



* * One adductor impression. 



PECTIKIDJB. Shell eared ... ... ... ... ... 81 



ANoainixffi. Inequivalve, lower valve with a hole or notch near 



the summit ... ... ... ... ... ... 83 



:. Inequivalve, lower valve entire ... ... ... 81 



Class Brachiopoda. 



Enclosed in a bivalve shell. 



(a.) Lower valve with an interior shelly plate. 



TEEEBEATULID^;. Shell minutely punctate ... ... ... 85 



EHYNCHONELLHXS;. Shell not punctate ... ... ... 87 



(b.) Lower valve without any shelly plate. 



CEANIAIXE. Ventral valve attached 87 



