ENTOMOCONCHUS. 47 



"Annulosa.'') The hinge-line is simple, the thin edge of the right valve being received 

 under the overlap of the opposite valve. 



The posterior portion of the carapace is rounded, the curve varying with individuals. 

 The anterior is truncate, usually obliquely, and with a more or less sinuous outline, due 

 to a depression accompanying the slight notch that is cut out (or rather indented) below 

 the well-marked antero-dorsal angle. These correspond with the hood and notch of most 

 of the Cypridinadce} The gape or opening at the notch is narrow and vertical (not 

 transverse, as in Cypridina and many of its allies). It is widest above, and closes at 

 about the middle of the vertical line, but reopens, with a smaller vertical fissure, at the 

 antero-ventral angle of the carapace, which is rounded and subcarinate, being impressed 

 on either side by a marginal furrow continued downwards from the depressed area. In 

 old individuals a short oblique furrow passes off on each side from the great sinus or 

 depressed area of the front of the carapace ; it is directed backwards and downwards, 

 from below the hood-like notch, and above the antero-ventral dehiscence of the valves. 

 There is also a small round or oval space left between the valves, sometimes accompanied 

 by a slight prominence at the postero-ventral angle, or at the corresponding curve. This 

 probably had relation to a marginal spike on each valve, such as is met with in many 

 bivalved Entomostraca. Near the middle of the inside of each valve, but rather nearer 

 the antero-ventral angle, a relatively large " Muscle-spot " is strongly marked in old indi- 

 viduals of E. Scoideri by a suboval patch of short radiating furrows within a much larger 

 sunken circular area. 



A local cloudiness of discoloration only is sometimes seen at this point on the outside 

 of the perfect valves, but by the loss of the exterior coating, from solution of the 

 carbonate of lime (a result of weathering), the radiate lines of the Muscle-mark are fre- 

 quently brought to view. These vascular rays of the Muscle-spot are transverse in the 

 middle, longitudinal at the extremities above and below, and at graduated angles between 

 on either side.^ 



The Muscle-spot was distinctly indicated by Prof. M'Coy in 1S39. Somewhat 

 similar radiating groups of linear " lucid spots " are observable in several published 

 figures of Cypridina (Baird and others). 



A difference of outline and of the number of radiating canals occurs in the figured 

 Muscle-spots of our specimens (figs. 3 ^, 4 c?, 5 (/) ; but we do not see how to assign these 

 differences as characters of sex, age, or variety. Eig. 3 d, showing the coarsest radii, is 

 a perfect cast of the Muscle-spot ; the others are seen by partial loss of the outer crust of 

 the valve. 



1 Cypridina, Philomedes, Asterope and Bradycinetus possess the anterior or antero-ventral notch 

 JEurypylus and Heterodesmus have it far less developed or barely present. Polycope, a member of an 

 allied group, has no notch. See above, pages 3 et seq. 



2 A curiously similar pattern is seen on the dorsum of Cryptonota citrina, Stimpson, ' Invertebr. 

 Grand Manon,' p. 36, pi. 2, fig. 27. 



