PIIYLOGENY OF THE PELECYPODA. 307 



but it. may not be amiss to repeat the observations as my specimens, growing on transparent 

 glass, were very clear. In I'l. xxiv, fig. 4, the dotted outlines of the alimentary canal are 

 those portions not visible from the right side, but seen when the oyster was viewed from 

 the left side through the glass microscope slide to which it was attached. From the 

 upper part of the hood-shaped palps a short oesophagus, ce, leads directly .into the stom- 

 acu, sf. The stomach, embedded in greenish-brown liver, has a circuitous outline espe- 

 cially on its ventral boi'der. The intestine leads from the posterior end of the stomach 

 and extends in a nearly dii'cct line to a ])oint half way past the adductor muscle. It then 

 takes a sharp bend and, returning on itself, curves and follows the dorsal outline of the 

 body; again turning Avhen near the umbos, it follows the outline of the body and passes 

 to the left of the oesophagus. It is then produced posteriorly in a curving line, passing to 

 the left at the two points where it crosses the earlier portion of its own track and emerges 

 from the liver mass just above the heart. It passes outside of the heart and wi'apping 

 itself around the adductor muscle terminates in close relation to that muscle. This 

 course of the alimentaiy canal is identical with that figured ))y Rydei* (60), in an adult, 

 excepting that in the adult the oesophagus is directed more anterioi'ly than in the spat. 

 The ffEces, of a peculiar, nearly horse-shoe shape,' pass from the intestine in small ])ai'- 

 ticles and arc carried from the cloacal space by the cxcurrent water from the gills 

 through the opening, s, PI. xxiv, fig. 4, formed by a sinuosity of the border of the i-ight 

 valve. All water from the body passes out through this opening, as indicated by arrows, 

 and the mantle is retracted to allow of its i)assage. 



A special excurrent passage is characteristic of younger and older spat than the one 

 described (PL xxv, figs. 5-6) ; but no trace of it has been found in the adult. The sinu- 

 osity of the right valve is apparently due to the constantly I'etracted condition of the 

 ]-ight mantle border at this point. It is constantly retracted to admit of the passage of 

 the excurrent water and being so retracted, as a necessary result, the shell is not built 

 at this area as i-apidly as at other places where the mantle is fully extended. It might 

 be considered that the peculiar excurrent opening was due to the mechanical conditions 

 inider which the oyster lives, an opening being necessary on account of the close rela- 

 tion of the shell to tlie object of fixation, and it is possible that this is the ease; but 

 against it may be ui-ged tlie fact that no such character is foinid in Anomia which is sub- 

 ject to similar conditions. I think that this excurrent passage and the correlated sinu- 

 osity of the right valve are of hereditary significance. Examiningthefossils, we findasim- 

 ilar sinuosity of the left and right valve characteristic in marked degi'ee of the genus 

 Gryphfca; it is also found frequently in Exogyra and in many fossil members of the Avic- 

 ulida;, from which last group the Ostreadte were doubtless evolved.^ 



As in the case presented, many peculiarities of the form of shells can doubtless be 

 elucidated by a study of the soft parts and life-habits of the animals. The byssal notch, 

 ridges of large siphonated forms as the Myida>, and siphoiiate processes of Gasteropoda 

 are examples of such characters, many of which are specially studied in this paper. 



• The horse-slioe shape is the form of the cross sec- forms in the line of cvohition of the group. I do not 



tion of tlie ficces and owes its origin to the peculiar form think this militatas against what is above stated. Osti'ea 



of the intestine Ryder (00). has an excurrent sinuosity of the valves similar to tluit 



'In section VII, it is stated that Grypha'a and E.-iogyraare found in Grypha^a and Exogyra, inall, inherited (if atall) 



prolialjly aberrant members of the Ostreada', not tyi>ieal from far-oft' ancestral groups. 



