MONOMYAIRA. 



15 



beaks ; ligament subexternal, short, placed in the sloping area, 

 at the top of the valves. 



Approximating in form to the Perna, but very different in the form of its 

 hinge, and in tliis respect more nearly resembling the Avicula, though it 

 cannot be mistaken for it, both being without the sulcated teeth or joints at 

 the hinge ; but the conical cavity, situated under the beaks of the Malleus, 

 and crossing the angle of the slope of the ligament, at once distinguishes it 

 from the Avicula. 



Malleus albus. — The White Malleus. Plate XV. fi?. 12. 

 Trilobate ; base of the lateral lobe prolonged, without a sinus, 

 the base and the pit for the ligament not distinct ; white, with 

 transverse undulations. Four inches long. Inhabits the Aus- 

 tralian seas. 



Genus 25.— PERNA.— Za?«fi!rcA'. 



Generic Character. — Subequivalve, flattened, slightly distort- 

 ed, of a lamellar texture; hinge linear, marginal, and many 

 toothed ; these are furrow-like, parallel, and transverse, not 

 inserted in the opposite furrows, between which the ligament is 

 inserted ; sinus for the byssus somewhat gaping, and placed un- 

 der the extremity of the hinge. 



The substance of the shell, although solid, is formed of flaky portions, not 

 adhering closely to each other, and giving it a foliaceous appearance. 



Perna ephippium. — The Saddle Perna. Plate XV. fig. 

 1.3. Compressed, suborbicular ; left valve produced, the margins 

 acute ; purplish-brown on the outside, and pearly within. Four 

 inches long. Inhabits the Indian seas. 



Genus 26.— CRENATULA.— Zamarcyt. 



Generic Character. — Subequivalve, flattened, somewhat dis- 

 torted and lamellar ; with an opening for the byssus ; hinge 

 lateral, linear, margined, and crenated ; the crenulations in a 

 regular series, and somewhat hollowed for the reception of the 

 ligament. 



The distinction betwixt this genus and the Perna, consists in the hinge of 

 the Crenatula being composed of slightly concave callous crenulations, 

 which receive the ligament ; while in the Perna it consists of parallel trun- 

 cated linear teeth, corresponding and opposed to the opposite ones, the liga- 

 ment being inserted in their interstices only. 



Crenatula mytiloides. — The Muscle- Shaped Crenatula. 

 Plate XV. fig. H. Oblong-ovate, oblique; the base acute; 



