384 ROBKllT TRACY JACKSON ON THE 



observed. The numerous and peculiar cartilage pitts, loss of hinge teeth and form of the 

 shell ])oint out clearly that Perna is a side branch, oif the ascendent line of the Aviculan 

 series. In these studies Perna is considered the main stem from which important 

 branches are given off. (Further description of Perna, see pp. 326, 327.) 



Bakewellia appears to be a branch from Avicula, in the same line of variation as Perna, 

 on account of the series of cartilage pits. Tlie form of adult Bakewellia is very similar 

 to the yoimg of Perna, Pl.xxvii, fig. 1, so that it might appear to be in the line of ancestry 

 of Perna. An objection to this assumption is the high d(;velopment of both adductor 

 muscles, whereas neither Avicula nor Pei'ua has more than a slight development of the 

 antei'ior adductor. A dimyarian condition has been shown to be characteristic of the 

 prodissoconch stage of Ostrea, Pecten, Avicula, etc., and in Bakewellia, we merely have to 

 assume that the early anterior adductor was retained instead of abandoned in the ontogeny 

 of the individual. A similar case of the retention of the anterior adductor is seen in 

 Dimya, which has two adductoi'S in the adult though belonging to a typical monomya- 

 rian group. The form of Bakewellia is sulj-rhomboidal, oblique and is quite comparable 

 to Rhombopteria. The cartilage pits, primitive form of the valves and large anterior 

 adductor muscle, point to the conclusion that Bakewellia is a specialized and partially 

 retrogressive genus. 



Gervillia has important characters of foi'm and cartilage pits connecting it with Perna 

 from which genus it was probably derived. Hornesia, an allied genus, is apparently a 

 modified branch from Gervillia diifering principally in its iuequivalvular form. 



Inoceraraus was probablj' derived from Perna, through the intermediation of Gervillia. 

 The forms of the genus Avhich are rhomboidal and the most equivalvular, as Inoceramus 

 cris2)l, Mant., are to be considered the more normal, least modified forms of the genus as 

 they approach nearest to the condition found in ancestral genera. The oval and pli- 

 cated genus Actinocei'amus is a modified branch from Inoceramus. Yolviceramus is a 

 still more modified, oval and highly involute genus probably derived from Actinocera- 

 mus. 



Pernostrea from the Jurassic is a descendant of Perna which attached itself by the 

 umbo of the left valve and the consequent conditions of fixation modified the shell, in- 

 ducing an ostrean form. The shell has a series of cartilage ])its like Perna and a form 

 which closely resembles an oyster. (Further description of Pernostrea, see j). 325.) 



From the studies on the development of the oyster, sections iv-vrr, we have come to 

 consider Ostrea as close akin to and derived from Perna (pp. 326-327). The points of 

 likeness are features in the anatomy of the soft parts, the prodissoconch and the struct- 

 ure of the right, free, least modified valve. The points of difference are the absence of 

 the foot, the highly modified attached valve and the single cartilage pit of Ostrea. The 

 first two leatures of difference are paralleled in Pernostrea and are ascribed to the effects 

 of the condition of cemented fixation (section vir). The oyster is a highly modified 

 form and the reduction of the several cartilage pits of Perna to a single one in Ostrea 

 is considered as jiai-t of the specialization of the genus. When young, Perna also has 

 but a single cartilage pit like the oyster; compare fig. 30, p. 329 with PI. xxiv, fig. 19. 

 Ostrea is the least modified member of the Ostread;i3, as it nearest appi'oaches to Perna 

 from which it was apparently derived. Grypha3a is a more highly modified genus and 



