MYTILIDJE. 57 



MYTILID^S. 



We comprise in this family the genera Mytilus, Modiola, 

 Crenella, Pinna, and Avicula. In coming to this arrangement 

 we have had difficulties to contend with. At one time we 

 were inclined, in consequence of M. Deshayes' remarks, to 

 include Modiola and Crenella in Mytilus-, but having very 

 lately examined species of the three genera, we are by no 

 means satisfied of the propriety of such a procedure. We 

 think, from the configuration of the branchial laminae and the 

 partly closed mantle in Crenella, it must stand as a genus. 

 With respect to Modiola, our accounts of two of its species 

 show that there are in them considerable variations : this cir- 

 cumstance, with the pinnated character of the mantle in My- 

 tilus and the differences of the branchial and anal mantellar 

 terminations of the two genera, have made us hesitate to 

 merge Modiola in Mytilus. The variations in the two do not 

 perhaps amount to generic ones, and a coalition may even- 

 tually take place. We have no difficulty in placing Pinna 

 with the My tili ; its animal in almost all points, particularly 

 in the shape and position of the adductor muscles and internal 

 nacreous aspect of the shells of the two genera, appears to 

 support this determination. With Avicula we have greater 

 hesitation ; but the recorded accounts by Poli, of the animals 

 of that genus and Pinna, show such a similitude as to consti- 

 tute a primd facie case for depositing them with the Mytilida. 

 With these views, the family of the Aviculada may be dis- 

 pensed with. This location of Avicula must not be con- 

 sidered as settled, even with reference to our method, for 

 it is not improbable that it may have considerable alliance 

 with the Ostreadte ; but it is really immaterial as regards 

 natural order whether it ranges with the Mytili or Ostrece, or 

 constitutes an independent family, for in that case it must be 

 fixed between the Mytilida and Ostreadce. 



We would recommend to naturalists a strict re-examination 



