45 



MONOGRAPH OF THE GENUS 

 PECTEN, 



Char. Gen. Molluscum acephalopodum monomyarium. Testa 

 interdum libera, insequivalvi, subtequilaterali, biauriculata, 

 plerumque radiatim costata, atque regulari ; cardine recta, 

 foveola trigona. centrali cartiliginem reeipiente; ligamentum 

 lineare ; umbonibus contiguis. 



Obs. The bivalve shells included in this genus are free, 

 regular, inequivalve, subequilateral, with a pair of triangular 

 flattened auricles forming the hinge margin in each valve ; 

 and ribs radiating from the point of the umbones. The hinge 

 is straight and toothless, with a linear ligament near the edge, 

 and a triangular pit in the centre, containing the elastic sub- 

 stance called the cartilage. In nearly all the species, there 

 is a sinus in the anterior auricle of the lower, or right valve, 

 apparently to admit the passage of a byssus. 



In nearly all cases the colouring is much paler in the right 

 valve (therefore considered the lower) than in the left, and in 

 many instances the rich tints and markings which adorn the 

 latter, are entirely wanting in the former. 



DESCRIPTIONS. 



Sp. 1. P. maximus. (pi. xv. f. 98 to 100.) Linn. Syst. 

 Nat. p, 1144. 



T. subrotundata, lateraliter expansa, subdepressa, ; umboni- 

 bus laevibus, marginibus striatis ; valva sinistra prope umbonem 

 concava, prope mediam paululum elevata ; auriculis magnis, 

 rectangulatis, striatis, cardine oblique plicato ; costis 12 ad 14 

 subdepressis, rotundatis, sulcatis, latis; interstitiis sulcatis ; 

 colore albo-lutescente, vel fusco-lutescente, fusco vel rubro 

 precipue ad umbones, angulariter maculato. Long. 6. 50.; lat. 

 1. 60. ; alt. 6.; poll. 



A large, rounded, inequivalve species, rather spread at the 

 sides, and having 12 or 14 rounded, sulcated, rather flattish 

 ribs in each valve. It is smooth near the umbones, and in the 



