SUBJECTS OK NATURAL HISTORY. 197 



in bituminized vegetable resins or amber, in the tertiary calcareous 

 slates, and in gypsum ; many fossil insects are found in the brown 

 coal and bituminous marl slate. The accordance of these extinct 

 forms with those now existing, and the narrow limits of their vari- 

 ations, shew in an extraordinary manner the unity of plan which 

 pervades every department of the animal kingdom. We find just 

 the same system and harmony in the laws which regulated the form- 

 ation of these beings thousands of ages ago as are now manifested. 



The Crustacea may be called the spiders of the sea : the Lobster, 

 Crab, and Shrimp are sufficiently known to you. Many of the spe- 

 cies in this class are very agile and sprightly : and although the 

 mode of progression in the Lobster and the Shrimp is tail foremost, 

 still they are capable of taking an extensive spring, the former 

 bounding to a distance of thirty feet by one or two sweeps of the 

 tail. Perhaps the most interesting fact I can mention respecting 

 these crustaceous animals is that of their periodically casting off 

 their shelly covering. The old shell is removed in detached pieces, 

 the animal escaping from it with a soft newly-formed epidermis, 

 which soon secretes the earthy matters to form a new shell, thin 

 layers being deposited in succession until the animal has formed for 

 itself an entire jointed sheath, corresponding with the increased 

 magnitude which it assumes at each fresh casting of the shell. 

 Many interesting forms of fossil Crustacea are found in the London 

 clay. The crustaceous animals complete the second grand division 

 of the animal kingdom. 



The simplest family of the molluscous animals is the Tunicata ; 

 so called from their soft transparent external covering. These ani- 

 mals are closely allied in their structure to those inhabiting bivalve 

 shells. They are often found thrown upon the shore by the agi- 

 tated states of the sea. The Tunicata frequently cover themselves 

 with an adventitious solid earthy matter formed of particles of gra- 

 vel, shells, or mud. The beautiful little Pi/rosoma, myriads of 

 which contribute to illumine the ocean at night, especially in tropi- 

 cal regions, is one of the class Tunicata. 



The Conchifera include all the inhabitants of bivalve shells. In 

 the study of Zoology, as now pursued, these shells and their archi- 

 tects must be considered together; and Conchology, as formerly 

 understood, can hardly be considered as forming any part of Natu- 

 ral History, unless we allow that it embraces what the French call 

 Malacology — that is a history of the animals which the shells inclose. 

 The Linnaean Conchology may be very useful to persons collecting 

 shells n>^ u^^..^;|'..^ oiijocts of creation ; but when we wish to study 



