624 MOLLUSCA CLASS ILPELECYPODA. 



principally inhabitants of southern waters, but are represented in the 

 Mediterranean. 



The ASTARTID^E, on the other hand, are northern forms. Their shells are 

 rather flattened, subtriangular, or ovoid in outline, marked with concentric 

 ribs, and covered with a thick brown periostracum. There is a strong hinge 

 with two or three cardinal teeth in each valve ; the ligament is external. 



The CRASSATELLID^E, another family of southern range, have a much longer 

 shell, generally subtriangular and concentrically ribbed ; there is a distinct 

 lunule ; the hinge is furnished with two or three cardinals in each valve ; the 

 resilium is lodged in a pit between the teeth. 



The ARCTICID^E are best known by the Arctica [ = Cyprina] Islandica of our 

 own northern shores. It has a strong, fairly globose shell, with prominent 

 umbones, covered with a rough brown periostracum ; there is no lunule. 

 The ligament is external ; the hinge teeth well developed, and, like those of 

 the Veneracea, there are two cardinal teeth, two posterial laterals, and two 

 feeble anterior laterals in each valve. Another genus, Libitina, is found in 

 the Red Sea and Indian Ocean ; it is an oblong shell with very long, straight, 

 posterior teeth. Another member of the group is Coralliophaga^ which, in 

 external form, closely resembles LithodomuSj already alluded to under 

 Mytilacea. 



The ISOCARDIID^E, or heart-cockles, which were formerly associated with 

 the preceding, have very globose shells, and very prominent, widely separated, 

 spiral umbones. The ligament which follows the umbones in their growth 

 is split in front and curved out on either side. 



A contrast to the foregoing is offered by most of the LUCINID^J, which 

 have very round, but rather depressed, and generally white shells, with 

 small umbones. The teeth, when well developed, comprise two cardinal and 

 one lateral in each valve ; the ligament is internal and very large. There is 

 generally, in addition to the usual muscular scars on the interior, the mark 

 of a muscular attachment running obliquely right across the valves. The 

 foot is long and cylindrical. The UNGULINID^E, with a single genus Ungulina, 

 are closely allied. 



The three remaining families, KELLYELLID.E, ERYCINID/E, and GALEOM- 

 MID.E are small shells, which have the common characteristic of a foot 

 adapted for creeping ; many of them when thus crawling spread the valves 

 wide open. The mantle is generally more or less reflected over the shell, 

 and in Ephippodonta and Clamydoconcha it completely and permanently 

 envelopes the valves. 



Sub-order 2. TELLINACEA. These have very long siphons, which are not 

 united ; the pallial sinus is correspondingly deep ; the foot is large and 

 flattened. 



In the type family TELLINID^:, or sunset shells, the shell is generally very 

 elongate, sometimes smooth and polished, sometimes marked with concentric 

 ribs, and oblique and gaping behind. There are two cardinal and two lateral 

 teeth, one anterior and one posterior, in each valve ; the ligament is external. 

 The Tellinidae are world-wide in their distribution, but the finer specimens 

 come from tropical seas. 



The SCROBICULARIID^ are Tellinse in which the ligament is internal ; they 

 love the mud of estuaries to dwell in, and obtain their food from it. 



The PSAMMOBIID.E resemble the Tellinidae both in shell and animal. ^The 

 former, however, is more equivalve, less twisted, and gapes more widely 



