MYTILIDA, 57 
MYTILID A. 
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 laminze 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 Mytili; 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 Mytilide. 
With these views, the family of the Aviculade 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 Ostreade; but it is really immaterial as regards 
natural order whether it ranges with the Mytili or Ostree, or 
constitutes an independent family, for in that case it must be 
fixed between the Mytihde and Ostreade. 
We would recommend to naturalists a strict re-examination 
