Which of the following structures are present in the ventricles of the heart but not in the atria?

Chordae tendineae (chord- =cord; tend- = tendon) are tendon-like cords that connect the cusps of the atrioventricular valves to papillary muscles. They prevent bulging of atrioventricular valves too far into the atria. They are confined to ventricles and are absent in the atria.

Cardiomyocytes (cardiac muscle fibres) and nodal tissues are located in atria and ventricles. 

Pectinate muscles are confined to atria. They are the muscular ridges on the inside of anterior wall of right atrium and right and left auricles.