LOWER SILURIAN LAMELLIBRANCHS. 637 



the byssal sinus, and terminating at a point on the inner side of the 

 umbonal cavity. Anterior muscular scar not observed, apparently want- 

 ing. A deep pedal muscle scar situated a short distance behind the beak 

 and partly under the hinge plate. 



Type: A. gigantea, S. A. Miller. Plate 50, figures 1 to 4. 



Plate 50, figures 1 to 4. 



The above description is based upon a fine series of specimens be- 

 longing to m}- private cabinet and upon the original types of the genus 

 which were kindly loaned for the purpose \>y Dr. S. A. Miller. I have 

 endeavored to give the facts without prejudice and just as they appear 

 to me after a careful study of the whole family to which the genus be- 

 longs. This statement is necessary considering the fact that some points 

 in Dr. Miller's original description were questioned \>y Dr. C. A. White, 

 notably the position of the muscular scars.* 



I shall not ; however, enter into the discussion carried on by these 

 gentlemen except to say that Dr. Miller is certainly in error when he says 

 that there is an "anterior muscular scar below the byssal sinus." The 

 depressed subtriangular space ( see plate 50, fig. 4), which he mistook 

 for a muscular impression is without doubt due to some abnormal thick" 

 ening of the internal surface of the valve. Nothing of the kind has 

 been observed in any other of the numerous specimens seen by me, while 

 the true position of the large muscular scar, which was left by the pos- 

 terior adductor and not the anterior, is unequivocally shown in several 

 cases. The pallial line also is clearly shown in the specimens, and as it 

 runs through the space to which the muscle was supposed to have been 

 attached and on to the cavity of the beak, it affords the very best evi- 

 dence in favor of the view here adopted. 



As understood by me Anomalodonta agrees in all esssential respects, 

 except the hinge, with the new genera Byssonychia and Eridonychia, and 

 as far as the muscular impressions and the pallial line are concerned, with 

 all the genera now referred to the AmbonychiidcE. The absence of true 

 cardinal and lateral teeth sufficiently distinguishes the genus from Bys- 

 sonychia. In Eridonychia, however, the hingement is very similar to 

 that of Anomalodon'a; 3 T et it differs and is even more simple in wanting 

 the peculiar oblique fold at the anterior extremity of the hinge plate. 

 The latter is also shorter and not so strong, and the shells more oblique. 



Beside A. gigantea there are only two or three species that may be 

 referred to Anomaloaonta. These are the Ambonychia alata, Meek, a new 

 species, and possibly the little known Ambonychia costata, Meek; and all 

 of them are found only in the middle and upper beds of the Cincinnati 

 group. The generic type seems therefore to have been a limited one in 

 every sense. 



*See Cincinnati Quarterly Journal of Science, Vol. I, p, 326, 1874; and Vol. II, 

 D. 280, 1875. 



