The points from which the growth of the shell commences, are called the _umbones_; these usually turn towards the anterior part of the shell: if this circumstance fails to point out the anterior, it may in many cases be distinguished by the muscular impressions of the mantle. If this has a sinus or winding, it is always near the posterior muscular impression; and in all cases where there is an external ligament, it is on the posterior side. There is sometimes an impression near the front of the umbones, which forms a semicircle on each valve; the space within this semicircle is called the _lunule_ (wood-cut, fig. 71 and 72, _l. l. l._); a corresponding depression, when it exists on the posterior margin near the umbones, is named the escutcheon. _Hinge._ [Illustration: Fig. 73, _l t_, lateral teeth; _c t_, cardinal teeth; _c_, cartilage under the ligament; _l_, ligament; _f_, fulcrum of the ligament.] The _hinge_ of the shell is on the _dorsal_ margin, and is composed of the various apparatus by which the two valves act upon each other in opening and shutting. It consists of a _ligament_, which is placed on the dorsal margin, just at the back of the umbones, and unites the two valves together; the _cartilage_ or thick gristly elastic substance, sometimes found close to the _ligament_, to which it then forms an inner coating, and sometimes received into a pit within the shell. It serves the purpose of keeping the shell open when not forcibly closed by the adductor muscles. An inner layer of shelly matter upon which are placed teeth, and pits to receive them on the two valves reciprocally. Each of these it will be necessary to treat of more at large; observing, at the same time, that in some species of Bivalves these parts may be wholly or partially wanting. Thus we meet with some shells, such as the Muscle, without teeth; and there is the group containing Pholas, &c. the hinge of which is destitute of teeth and ligament, the two valves being kept together by loose cartilages, and by the contracted space in which they are confined. _Ligament and Ligamentary Cartilage of the Hinge._