23S ■ EDWARD S. MORSE ON 



the stomach, while the laterahs are turned sharply to the right and left respectively. 

 The anterior occlusors are shortened in their longituchnal diameter. In 47 : 6, the lateralis 

 muscles are drawn out, exposing a much longer section of the oesophagus and clearly 

 exposing the pharyngeal glands, and the anterior portion of the stomach is no longer 

 bifurcated, the anterior occlusors being also drawn out longitu.Unally. If these prrts 

 have a limited motion, these various appearances are accomplished mainly by a contrac- 

 tion of the obliquus externus, obliquus medius, and lateralis. In the contraction of the 

 lateralis, the dorsal shell with its attachments is drawn backward, as the ventral shell is 

 rigidly held by the peduncle, producing the appearance seen in 47 : 5, while the contrac- 

 tion of the obliquus externus and medius draws the dorsal shell forward, presenting the 

 appearance shown in 47: C From this description one may get an idea of the mobility 

 of the dorsal shell. 



There is but little to add to what has already been made known concerning the mus- 

 cles of D. lamdlosa. It seems strange, however, that Owen, Joul)in, and others should 

 not have seen the true character of the anterior occlusors. Blochmann is the first to 

 give a correct figure of these muscles, in which he has shown that the occlusor muscle is 

 really divided into two muscles, which he indicates as anterior occlusor 1 and 2. In my 

 stuches of these muscles I had intended naming them external and internal occlusors 

 respectively, but to avoid increasing the abeady confusing nomenclature on the subject, I 

 shall adopt the names given by Blochmann. In L. Jepldula, the separation of the ante- 

 rior occlusor into two elements has just been pointed out, and it would seem that they 

 must be homologous with these muscles in D. lamdlosa. The lateralis in Lingula and 

 Discinisca is also found where it might l)e expected, considering that in lingula the body 

 is drawn out while in D. lamellosa it is concentrated. A further reference will be made 

 to the musculature of Discini.sca in a consideration of the early stages of the animal. The 

 muscles of Terebratulina and allied forms have l^een thoroughly described by Hancock, 

 and my observations are only confirmatory of what is already known. The interlocking 

 shells of the Testicardine forms would seem to preclude any other function of the mus- 

 cles save that of the opening and closing of the shells, and of causing the body to move 

 on the peduncle. In Discinisca, the silvery and tendinous muscles connected with the 

 peduncle imply much freedom of motion of the body. In the Testicardines the nuiscles 

 have, without exception, bi-oad bases of attachment tapering toward the middle of the 

 muscle, while in the Ecardines, with the exception of the lateralis and the oliliquus 

 medius, the muscles have the same diameter throughout. In the young of D. lamellosa 

 (61 : 11 ) , however, tlie muscles have broad bases of attachment. A very marked difference 

 therefore is seen in the muscles of the Testicardine and Ecardine groups. 



