294 mb. r. g. parsons on the [Mar. 20, 



except in Ceredon, iu which a small adductor secundi digiti was 

 found, but no adductor miniini digiti. 



General Summary. 



The amount of facts at my disposal does not, of course, justify 

 anything like an attempt at a definite and complete summary of the 

 muscles of the Hystricomorpha and Sciuromorpha. The following 

 generalizations are merely suggested for future investigation. 



A. Differences between the Hystricomorpha and Sciuromorpha. 



1. In the Hystricornorpha the anterior deep part of the masseter 

 passes through the infraorbital foramen. In the Sciuromorpha it 

 does not. 



2. Iu the Hystricomorpha, with the exception of the Dipodidse, 

 the digastric has no complete division into two bellies, and the 

 muscles of opposite sides do not communicate. In the Sciuro- 

 morpha, as well as in the Dipodidse, a tendon completely divides 

 the two bellies, and the muscles are connected across the middle 

 line by a tendinous arcade. 



3. The transverse mandibular muscle is absent in the Hystrico- 

 morpha, with the exception of the Dipodidas. It is present in the 

 Sciuromorpha, with the exception of Castor. 



4. The genio-hyoid muscles of opposite sides coalesce posteriorly 

 in the Sciuromorpha, but not in the Hystricomorpha. 



5. The oino-hyoid is present or absent in the Hystricomorpha. 

 It is always present in the Sciuromorpha. 



6. The levator claviculae rises either from the atlas or the basi- 

 occipital in the Hystricomorpha. Always from the atlas in the 

 Sciuromorpha. 



7. The scalenus anticus is present in the Hystricomorpha, except 

 in the Hystricidae. It is absent in the Sciuromorpha. 



8. The trapezius is often divided into an anterior and posterior 

 part in the Hystricomorpha. Never in the Sciuromorpha. 



9. The sterno-scapular muscle is composed of the subclavius 

 and the scapulo-clavicularis in the Hystricomorpha. In the Sciuro- 

 morpha, as well as in the Hipodidic, only the subclavius is present. 



10. The first part of the coraco-brachialis (rotator humeri) is 

 always present in the Sciuromorpha. In the Hystricomorpha it 

 is present or absent. 



11. The pronator quadratus is always attached to more than a 

 third of the bones of the forearm in the Hystricomorpha. In the 

 Sciuromorpha it is attached to a third. 



12. The supinator longus is present in all the Sciuromorpha 

 except Castor. It is absent in the Hystricomorpha except in 

 Erethizon and the Dipodidse. 



13. The scansorius is always wanting in the Sciuromorpha. It 

 is often distinct in the Hystricomorpha. 



14. The quadratus femoris usually has a tendinous insertion in 

 the Hystricomorpha. It is fleshy in the Sciuromorpha. 



