CRENATULA. 



It may be known from Area by its irregularity, its partly 

 internal ligament placed between the crenulations of the 

 hinge, and by its foliaceous texture. We do not know 

 any other Genera with which the Crenatulae can be con- 

 founded : its near resemblance to Perna will render it 

 necessary as we proceed to mention every particular 

 difference between the two Genera, which we shall do in 

 describing Crenatula ; and this appears to us the more 

 necessary, as Bowdich's representation of a shell of this 

 Genus is not calculated to give even a tolerable idea of 

 the subject intended ; and as Parkinson seems to have mis- 

 taken a small species of Perna for one of this Genus, 

 evidenced by his shewing distinctly the sinus for the pas- 

 sage of the byssus. 



The Crenatulae are nearly equivalve, flat, lamellose, 

 rather irregular, and similar in these characters to Perna, 

 though rather more gibbous. The hinge of Crenatula is la- 

 teral (because the shell itself is obliquely elongated, which 

 Perna is not,) linear, marginal, crenulated on the inside, 

 the crenulae arranged in a row along the hinge, each of 

 them forming a little rounded callosity and hollowed out 

 to receive a portion of the ligament. In several of these 

 characters it differs from Perna, whose hinge is, indeed, 

 marginal and linear; but instead of teeth or crenula- 

 tions, it is divided by more or less numerous transverse, 

 parallel grooves, (called dents sulciformes^ by Lamarck,) 

 which receive the principal portion of the ligament; for 

 the ligament in both seems to be divided into two prin- 

 cipal portions, one of which is situated in the grooves 

 in Perna and in the crenulations in Crenatula, and the 

 other being the smaller, is attached to the raised inter^ 

 stices in both. The Crenatulae are in general,^ we believe 

 always, fragile and slender shells, whereas the Pernae are 

 much stronger and of thicker substance; this difference 

 might be supposed to arise from age, but it is constantly 

 the case even in young specimens of Pernae. The pearly 

 portion of the inside of these shells differs also very mate- 

 rially, for in Perna it is larger and more extended than in 

 Crenatula, taking up the greater part of the inside and 

 remaining of th^ same shape as the shell, while in Crena- 

 tula it does not take up quite half of the inside of the 

 shell, it extends along the hinge margin, and forms nearly 

 a straight line from the umbo to the most distant point of 

 the sheJl, in an oblique directionj In Crenatula the mus- 



