396 CRETACEOUS PELECYPODA 



oval, generally liiglier than long, structure of the outer surface lamellar, beaks 

 anterior, with a small or obsolete anterior ear and a deep byssal sinus below it ; 

 hinge line straight, more or less thickened and flattened, with numerous sub-parallel 

 transverse, ligamental grooves, posterior muscular scar large, sub-central, oval or 

 rounded, anterior scar obsolete, or only indicated by a small pit; type, MeL 

 ephippmm, Linn, Eeeve describes (Icon., xi,) 28 recent species ; about six have to 

 be added to his list. Most of them are from the Eastern seas, but a few occur 

 in America. Eossil species are found in all sedimentary deposits, and their number 

 appears gradually to increase up to the present date. 



c. Sub-family— rULSULZIJVy^. 



Ligament lodged in a special, single groove, or pit, extending from the beak 

 internally. 



26. JEligmus, Deslongch., 1856, (Mem. d. 1. Soc, Linn, de Normandie, x, 

 p. 272). Elongately oval, equivalve, tumid, very inequilateral, beaks small, 

 sub-anterior, more or less close together, turned towards the front, with an 

 elongated ligamental pit, lunular* margin in front of the beaks undulating, 

 an irregular gape being produced by a number of incisions on it; muscular 

 scar single, situated on a raised plate, which extends obliquely and anteriorly 

 from below the beaks, outer surface generally radiately ribbed ; type, JE. polytypiis, 

 DesL, from Jurassic rocks of Normandy. Two other species have been described 

 from the same deposits by the same author, and one, J?, contorhts, by Laube, 

 also from Jurassic beds at Balin (Galicia). 



The outer lamellar as well as the inner somewhat nacreous layer of the shell 

 of Migmus are rather thin, their structure more resembling that of Ostrea than 

 of Vulsella, but the other characters relating to equal and free valves, the presence 

 of a gape in front of the beaks, which most probably corresponds to the byssal 

 sinus of other allied genera, and the ligamental groove, appear much better to 

 agree with Vulsella than with Ostrea, 



Munier-Chalmas discussed this question in an able notice published in the 

 Bull. d. 1. Soc, Norm., 1862, vol. viii, p. 97, but Deslongchamps (1. cit., p. Ill,) 

 does not accept either that author's or Deshayes' views, and maintains his own, 

 that Migmiis must be classed with Ostrea, As far as I have had opportunity to 

 examine several well-preserved specimens of the Balin and the Normandy Migmi, 

 I believe, that Deslongchamps is correct in rejecting the opinion, that the inner 

 layer of the shell of JEligmus has been removed by fossilisation. Neither the thick 

 internal pearly layer, nor the outer, strong, lamellar and partially fibrous layer 

 of Vulsella, are developed in Migmus to a large extent, but both appear to be 

 indicated. The differences in the structure of the shells of Migmus and Vulsella 

 have their perfect parallel in those of Crenatula and Melina f=PernaJ, and, I 

 think, therefore, that Deshayes and Munier are justified in classifying the two 



* Judging from analogy of other shells, I am rather disposed to regard the longer side of Migmus towards 

 which the beaks are incurved as the anterior one ; this view is also supported by the byssal opening in front of it. 



