METAZOA MOLLUSCA. 211 



in limestone rocks (No. 410, P. corditoides). The teeth of 

 the hinge are usually broken off, but when preserved there 

 are two in each valve. The mantle lobes are united. 

 The anterior end is short and the posterior gaping. 



Mactra (= Spisuld) solidissima Dillw. (No. 411), is 

 one of our largest New England bivalves. The animal is 

 free-moving and is therefore symmetrical. The horny 

 layer is light brown in color (Nos. 411, 412). The umbos 

 are directed anteriorly. The pit, with its internal liga- 

 ment, and the external ligament are sometimes developed. 

 The cardinal tooth is nearly vertical while the marginal 

 teeth are long. The dissection (No. 413) and model 

 (No. 412) show the principal internal organs and give 

 their names. The mantle is seen extending along the 

 edge of the shell (Nos. 412, 413) ; it has three openings; 

 one at the anterior end through which the foot passes out, 

 and two at the end of the siphon. This siphon is really 

 a double tube; one, "the branchial siphon," is for the 

 ingoing current of water, and the other, "the cloacal 

 siphon," for the outgoing current. The anterior and 

 posterior adductor muscles, the gills injected with red 

 coloring fluid, the mouth organs called palpi, and the 

 strong muscular foot are all shown in the preparation. 



Psammobia vespertina Gmel. (No. 414), has two long 

 delicate siphonal tubes separate throughout their whole 

 length. The siphonal tubes are separate only a short 

 distance in Psammosolen strigillatus Linn. (No. 415). 

 The foot in this genus is of great size and strength. 



Ensis directus Con. (Ensis americana Verr.) (Nos. 416- 

 418), and the European species, Ensis ensis Linn. (No. 

 419) have a long razor-like shell covered when young (No. 

 416) with a glossy horny layer, but which is often partly 

 worn off in the adult (No. 417). This animal travels 

 rapidly through the sand as can be proved by any one 

 who attempts to dig it up. Notwithstanding this fact, the 

 horny layer is seldom found entirely worn off. The 

 model (No. 418) shows the adult animal with the mantle 



